Add Yang model for SYSTEM_DEFAULTS table (#15129)

Why I did it
This PR is to backport PR #11117 into 202205 branch.
This PR is to define Yang model for SYSTEM_DEFAULTS table.
The table was introduced in PR sonic-net/SONiC#982
The table will be like

"SYSTEM_DEFAULTS": {
        "tunnel_qos_remap": {
            "status": "enabled"
        }
}
Work item tracking
Microsoft ADO (https://msazure.visualstudio.com/One/_workitems/edit/23037078)
How I did it
Add a new yang file sonic-system-defaults. Yang.

How to verify it
Verified by UT

Author: bingwang-ms

src/sonic-yang-models/doc/Configuration.md

@@ -28,7 +28,7 @@ Table of Contents
          * [DHCP_RELAY](#dhcp_relay)
          * [DSCP_TO_TC_MAP](#dscp_to_tc_map)  
          * [FLEX_COUNTER_TABLE](#flex_counter_table)  
-         * [KDUMP](#kdump)
+         * [KDUMP](#kdump)  
          * [L2 Neighbors](#l2-neighbors)  
          * [Loopback Interface](#loopback-interface)  
          * [LOSSLESS_TRAFFIC_PATTERN](#LOSSLESS_TRAFFIC_PATTERN)  
@@ -58,6 +58,7 @@ Table of Contents
          * [WRED_PROFILE](#wred_profile)  
          * [PASSWORD_HARDENING](#password_hardening)
          * [RADIUS](#radius)
+         * [SYSTEM_DEFAULTS table](#systemdefaults-table)
    * [For Developers](#for-developers)  
       * [Generating Application Config by Jinja2 Template](#generating-application-config-by-jinja2-template)
       * [Incremental Configuration by Subscribing to ConfigDB](#incremental-configuration-by-subscribing-to-configdb)
@@ -1801,6 +1802,31 @@ The RADIUS and RADIUS_SERVER tables define RADIUS config paramerters. RADIUS tab
 
 ```
 
+### SYSTEM_DEFAULTS table
+To have a better management of the features in SONiC, a new table `SYSTEM_DEFAULTS` is introduced.
+
+```
+"SYSTEM_DEFAULTS": {
+        "tunnel_qos_remap": {
+            "status": "enabled"
+        }
+        "default_bgp_status": {
+            "status": "down"
+        }
+        "synchronous_mode": {
+            "status": "enable"
+        }
+        "dhcp_server": {
+            "status": "enable"
+        }
+    }
+```
+The default value of flags in `SYSTEM_DEFAULTS` table can be set in `init_cfg.json` and loaded into db at system startup. These flags are usually set at image being build, and are unlikely to change at runtime.
+
+If the values in `config_db.json` is changed by user, it will not be rewritten back by `init_cfg.json` as `config_db.json` is loaded after `init_cfg.json` in [docker_image_ctl.j2](https://github.com/Azure/sonic-buildimage/blob/master/files/build_templates/docker_image_ctl.j2)
+
+For the flags that can be changed by reconfiguration, we can update entries in `minigraph.xml`, and parse the new values in to config_db with minigraph parser at reloading minigraph. If there are duplicated entries in `init_cfg.json` and `minigraph.xml`, the values in `minigraph.xml` will overwritten the values defined in `init_cfg.json`.
+#### 5.2.3 Update value directly in db memory
 
 For Developers
 ==============

src/sonic-yang-models/setup.py

@@ -108,6 +108,7 @@ def run(self):
                          './yang-models/sonic-extension.yang',
                          './yang-models/sonic-flex_counter.yang',
                          './yang-models/sonic-feature.yang',
+                         './yang-models/sonic-system-defaults.yang',
                          './yang-models/sonic-interface.yang',
                          './yang-models/sonic-kdump.yang',
                          './yang-models/sonic-loopback-interface.yang',
@@ -175,6 +176,7 @@ def run(self):
                          './cvlyang-models/sonic-extension.yang',
                          './cvlyang-models/sonic-flex_counter.yang',
                          './cvlyang-models/sonic-feature.yang',
+                         './cvlyang-models/sonic-system-defaults.yang',
                          './cvlyang-models/sonic-interface.yang',
                          './cvlyang-models/sonic-kdump.yang',
                          './cvlyang-models/sonic-loopback-interface.yang',

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -1954,6 +1954,11 @@
                 "TYPE": "RW"
             }
         },
+        "SYSTEM_DEFAULTS": {
+            "tunnel_qos_remap": {
+                "status": "enabled"
+	        }
+        },
         "LOSSLESS_TRAFFIC_PATTERN": {
             "AZURE": {
                 "mtu": "1024",

src/sonic-yang-models/tests/yang_model_tests/tests/system_defaults.json

@@ -0,0 +1,9 @@
+{
+    "SYSTEM_DEFAULTS_WITH_CORRECT_VALUES": {
+        "desc": "CONFIG SYSTEM_DEFAULTS TABLE WITH ALL THE CORRECT VALUES"
+    },
+    "SYSTEM_DEFAULTS_WITH_INVALID_STATUS_VALUE" : {
+        "desc": "Referring invalid status value",
+        "eStrKey": "InvalidValue"
+    }
+}

src/sonic-yang-models/tests/yang_model_tests/tests_config/system_defaults.json

@@ -0,0 +1,26 @@
+{
+    "SYSTEM_DEFAULTS_WITH_CORRECT_VALUES": {
+        "sonic-system-defaults:sonic-system-defaults": {
+            "sonic-system-defaults:SYSTEM_DEFAULTS": {
+                "SYSTEM_DEFAULTS_LIST": [
+                    {
+                        "name": "tunnel_qos_remap",
+                        "status": "enabled"
+                    }
+                ]
+            }
+        }
+    },
+    "SYSTEM_DEFAULTS_WITH_INVALID_STATUS_VALUE": {
+        "sonic-system-defaults:sonic-system-defaults": {
+            "sonic-system-defaults:SYSTEM_DEFAULTS": {
+                "SYSTEM_DEFAULTS_LIST": [
+                    {
+                        "name": "tunnel_qos_remap",
+                        "status": "invalid_status"
+                    }
+                ]
+            }
+        }
+    }
+}

src/sonic-yang-models/yang-models/sonic-system-defaults.yang

@@ -0,0 +1,39 @@
+module sonic-system-defaults{
+
+    yang-version 1.1;
+
+    namespace "http://github.com/Azure/system-defaults";
+    prefix system-defaults;
+
+    import sonic-types {
+        prefix stypes;
+    }
+
+    description "SYSTEM_DEFAULTS Table yang Module for SONiC";
+
+    container sonic-system-defaults {
+
+        container SYSTEM_DEFAULTS {
+
+            description "system_defaults table in config_db.json";
+
+            list SYSTEM_DEFAULTS_LIST {
+
+                key "name";
+
+                leaf name {
+                    description "feature name in SYSTEM_DEFAULTS table";
+                    type string {
+                        length 1..32;
+                    }
+                }
+
+                leaf status {
+                    description "default status of the feature";
+                    type stypes:admin_mode;
+                }
+
+            }
+        }
+    }
+}