add lacp_rate to portchannel (sonic-net#2036)

#### What I did

Make lacp_rate configurable for portchannel.


```
	Option specifying the rate in which we'll ask our link partner
	to transmit LACPDU packets in 802.3ad mode.  Possible values
	are:

	slow
		Request partner to transmit LACPDUs every 30 seconds

	fast
		Request partner to transmit LACPDUs every 1 second

	The default is slow.
```

#### Why I did it

In case of slow lacp_rate configuration link down will be detected in 60-90 seconds, it may be to long (for example for MCLAG high availability), in case of using ` --fast-rate=true` link down will be detected in 2-3 seconds.

#### How I did it
   
* add optional argument to `config portchannel` command, default=slow for backward compatibility. (this PR)
* parse argument in `teammgr` and forward it to `teamd` (other PR: sonic-net/sonic-swss#2121)
* update docs sonic-net/SONiC#937

#### How to verify it

Confgiure bond on other side, then configure portchannel and sniff the traffic from it.

```
config portchannel add PortChannel0001 --fast-rate=true
config portchannel member add PortChannel0001 Ethernet0
config interface ip add PortChannel0001  192.168.1.2/24
tcpdump -ne
```

Author: globaltrouble

config/main.py

@@ -1967,8 +1967,11 @@ def portchannel(db, ctx, namespace):
 @click.argument('portchannel_name', metavar='<portchannel_name>', required=True)
 @click.option('--min-links', default=1, type=click.IntRange(1,1024))
 @click.option('--fallback', default='false')
+@click.option('--fast-rate', default='false',
+              type=click.Choice(['true', 'false'],
+                                case_sensitive=False))
 @click.pass_context
-def add_portchannel(ctx, portchannel_name, min_links, fallback):
+def add_portchannel(ctx, portchannel_name, min_links, fallback, fast_rate):
     """Add port channel"""
     if is_portchannel_name_valid(portchannel_name) != True:
         ctx.fail("{} is invalid!, name should have prefix '{}' and suffix '{}'"
@@ -1979,9 +1982,12 @@ def add_portchannel(ctx, portchannel_name, min_links, fallback):
     if is_portchannel_present_in_db(db, portchannel_name):
         ctx.fail("{} already exists!".format(portchannel_name))
 
-    fvs = {'admin_status': 'up',
-           'mtu': '9100',
-           'lacp_key': 'auto'}
+    fvs = {
+        'admin_status': 'up',
+        'mtu': '9100',
+        'lacp_key': 'auto',
+        'fast_rate': fast_rate.lower(),
+    }
     if min_links != 0:
         fvs['min_links'] = str(min_links)
     if fallback != 'false':

doc/Command-Reference.md

@@ -6925,12 +6925,13 @@ When any port is already member of any other portchannel and if user tries to ad
 Command takes two optional arguements given below.
 1) min-links  - minimum number of links required to bring up the portchannel
 2) fallback - true/false. LACP fallback feature can be enabled / disabled.  When it is set to true, only one member port will be selected as active per portchannel during fallback mode. Refer https://github.com/Azure/SONiC/blob/master/doc/lag/LACP%20Fallback%20Feature%20for%20SONiC_v0.5.md for more details about fallback feature.
+3) fast-rate - true/false, default is false (slow). Option specifying the rate in which we'll ask our link partner to transmit LACPDU packets in 802.3ad mode. slow - request partner to transmit LACPDUs every 30 seconds, fast - request partner to transmit LACPDUs every 1 second. In slow mode 60-90 seconds needed to detect linkdown, in fast mode only 2-3 seconds.
 
 A port channel can be deleted only if it does not have any members or the members are already deleted. When a user tries to delete a port channel and the port channel still has one or more members that exist, the deletion of port channel is blocked. 
 
 - Usage:
   ```
-  config portchannel (add | del) <portchannel_name> [--min-links <num_min_links>] [--fallback (true | false)]
+  config portchannel (add | del) <portchannel_name> [--min-links <num_min_links>] [--fallback (true | false)  [--fast-rate (true | false)]
   ```
 
 - Example (Create the portchannel with name "PortChannel0011"):
@@ -11020,4 +11021,4 @@ ZTP will be restarted. You may lose switch data and connectivity, continue? [y/N
 Running command: ztp run -y
 ```
 
-Go Back To [Beginning of the document](#SONiC-COMMAND-LINE-INTERFACE-GUIDE) or [Beginning of this section](#ztp-configuration-and-show-commands)
+Go Back To [Beginning of the document](#SONiC-COMMAND-LINE-INTERFACE-GUIDE) or [Beginning of this section](#ztp-configuration-and-show-commands)

tests/portchannel_test.py

@@ -1,4 +1,5 @@
 import os
+import pytest
 import traceback
 
 from click.testing import CliRunner
@@ -60,7 +61,32 @@ def test_delete_non_existing_portchannel(self):
         print(result.output)
         assert result.exit_code != 0
         assert "Error: PortChannel0005 is not present." in result.output
-        
+
+    @pytest.mark.parametrize("fast_rate", ["False", "True", "false", "true"])
+    def test_add_portchannel_with_fast_rate(self, fast_rate):
+        runner = CliRunner()
+        db = Db()
+        obj = {'db':db.cfgdb}
+
+        # add a portchannel with fats rate
+        result = runner.invoke(config.config.commands["portchannel"].commands["add"], ["PortChannel0005", "--fast-rate", fast_rate], obj=obj)
+        print(result.exit_code)
+        print(result.output)
+        assert result.exit_code == 0
+
+    @pytest.mark.parametrize("fast_rate", ["Fls", "tru"])
+    def test_add_portchannel_with_invalid_fast_rate(self, fast_rate):
+        runner = CliRunner()
+        db = Db()
+        obj = {'db':db.cfgdb}
+
+        # add a portchannel with invalid fats rate
+        result = runner.invoke(config.config.commands["portchannel"].commands["add"], ["PortChannel0005", "--fast-rate", fast_rate], obj=obj)
+        print(result.exit_code)
+        print(result.output)
+        assert result.exit_code != 0
+        assert 'Invalid value for "--fast-rate"'  in result.output
+
     def test_add_portchannel_member_with_invalid_name(self):
         runner = CliRunner()
         db = Db()