[test-gap] Add a testcase to validate L2 switch config and functionality across reboots/upgrades

[vaibhavhd]

Description

1. Cold reboot a device.
2. Perform L2 config steps via L2 Switch mode · sonic-net/SONiC Wiki (github.com)
3. Install target image.
4. Cold reboot
5. Check config_db

Expectation: config db should not have configuration coming from minigraph (as there is no minigraph.xml).

Describe the results you received:

Describe the results you expected:

Additional information you deem important:

**Output of `show version`:**

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

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

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

[qiluo-msft]

This is to prevent potential bug in db_migrator.

[StormLiangMS]

hi @qiluo-msft this one seems a manage plane related test gap, assigned to you for further triage.

[qiluo-msft]

@vaibhavhd Do we really need this step?
Install target image.

I believe a reboot is good enough.