Add CLI to configure YANG config validation (sonic-net#2147)

**- What I did**
Add CLI to configure YANG config validation mode
`config yang_config_validation <enable|disable>`

**- How I did it**
Add a CLI script that writes the configuration of YANG config validation enable/disable into CONFIG_DB

Author: isabelmsft

config/main.py

@@ -1923,6 +1923,21 @@ def synchronous_mode(sync_mode):
     else:
         raise click.BadParameter("Error: Invalid argument %s, expect either enable or disable" % sync_mode)
 
+#
+# 'yang_config_validation' command ('config yang_config_validation ...')
+# 
+@config.command('yang_config_validation')
+@click.argument('yang_config_validation', metavar='<enable|disable>', required=True)
+def yang_config_validation(yang_config_validation):
+    # Enable or disable YANG validation on updates to ConfigDB
+    if yang_config_validation == 'enable' or yang_config_validation == 'disable':
+        config_db = ConfigDBConnector()
+        config_db.connect()
+        config_db.mod_entry('DEVICE_METADATA', 'localhost', {"yang_config_validation": yang_config_validation})
+        click.echo("""Wrote %s yang config validation into CONFIG_DB""" % yang_config_validation)
+    else:
+        raise click.BadParameter("Error: Invalid argument %s, expect either enable or disable" % yang_config_validation)
+
 #
 # 'portchannel' group ('config portchannel ...')
 #

tests/yang_config_validation_test.py

@@ -0,0 +1,33 @@
+from click.testing import CliRunner
+import config.main as config
+
+class TestYangConfigValidation(object):
+    @classmethod
+    def setup_class(cls):
+        print("SETUP")
+
+    def __check_result(self, result_msg, mode):
+        if mode == "enable" or mode == "disable":
+            expected_msg = """Wrote %s yang config validation into CONFIG_DB""" % mode
+        else:
+            expected_msg = "Error: Invalid argument %s, expect either enable or disable" % mode
+
+        return expected_msg in result_msg
+
+    def test_yang_config_validation(self):
+        runner = CliRunner()
+
+        result = runner.invoke(config.config.commands["yang_config_validation"], ["enable"])
+        print(result.output)
+        assert result.exit_code == 0
+        assert self.__check_result(result.output, "enable")
+
+        result = runner.invoke(config.config.commands["yang_config_validation"], ["disable"])
+        print(result.output)
+        assert result.exit_code == 0
+        assert self.__check_result(result.output, "disable")
+
+        result = runner.invoke(config.config.commands["yang_config_validation"], ["invalid-input"])
+        print(result.output)
+        assert result.exit_code != 0
+        assert self.__check_result(result.output, "invalid-input")