PortChannel mtu change fails with CLI

[anamehra]

Description
PortChannel mtu change fails with CLI
root@FC0:/home# config interface mtu PortChannel20 1510
Invalid port specified

Steps to reproduce the issue:

1. Configure a portchannel
2. modify mtu on portchannel

Describe the results you received:
Invalid port specified

Describe the results you expected:
The CLI should succeed and portchannel MTU should be updated

Additional information you deem important (e.g. issue happens only occasionally):
The CLI port changes are handled by /usr/bin/portconfig (sonic-utilities/scripts/portconfig) and as per current code, it handles it for only "PORT" table which is for port names starting with "Ethernet". It does not handle PORTCHANNEL table which handles the port names starting with "PortChannel".

This code needs to have handling like following at all locations where port tables are handled:
if port.startswith("Ethernet"):
port_tables = self.db.get_table(PORT_TABLE_NAME)
elif port.startswith("PortChannel"):
port_tables = self.db.get_table(PORTCHANNEL_TABLE_NAME)

**Output of `show version`:**

```

sonic-utilities @ c2c4c0d
```

**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```

[anshuv-mfst]

Issue seen in 201906, also seen on master as well.

[ghost]

Dear @anamehra, @gechiang,

I am preparing PR, which will bring support of PortChannels into portconfig script. Currently, I am checking unit tests of sonic-utilities for possibility to add new cases, according to the changes. Could I ask you to review sonic-net/sonic-utilities#1375?

[ghost]

Dear @anamehra, @gechiang,

I have checked unit tests and found there is no tests, related to config interface mtu or other subcommands of config interface.
I suppose a cause is described here: sonic-net/sonic-utilities#1301.

[anamehra]

Dear @maksymbelei95,
Diff looks good and works fine with EThernet and PortChannel ports. We would need this for 201911. Thanks

[ghost]

Dear @anamehra,

Thank you for review, I have added new PR for branch 201911: sonic-net/sonic-utilities#1384

Dear @gechiang,

Will be glad if you review PRs sonic-net/sonic-utilities#1375 and sonic-net/sonic-utilities#1384.
Thanks in advance.

[rlhui]

@gechiang would check