Fix for Db migrator & Vlan Changes

[sabakram]

What I did

This PR is Fixture for #3108. We have removed dbmigrator changes to avoid backward compatibility issues.

How I did it

Removed Dbmigrator changes from dbmigrator.py and dbmigrator_test.py

How to verify it

Dbmigrator changes has been removed

[sabakram]

@wen587 @qiluo-msft
Hi, we have removed DB migrator changes and catered to backward compatibility concerns in vlan.py. Can you please review this PR and suggest any other changes required to avoid backward compatibility issues. This is in reference to PR #3108 .

[wen587]

If "mode" not in port_data, the existing_mode should be undefined.
Why is it not caught by build or test?

[wen587]

Hi @sabakram , I tried submodule without your PR. sonic-net/sonic-buildimage#18480 It pass sonic-mgmt test. I suggest to revert it first to unblock submodule update.
You can have more time to improve the unit test to cover more use cases, especially the one used in test_vlan_interface.py

[sabakram]

Hi @wen587 , ok we can revert this PR to avoid blocking for submodule head. but this revert is already done once earler.
and this new PR was created totally after suggestions.
This case is being pending since long and every time all changes are done as per suggestions from start but still we have open PR for same case which is also causing blocking issues and compatibility issues. It would be great help if you reviewed PR and provide suggested changes again at once so we can re do all code to resolve this case.

[wen587]

Hi @sabakram , sure I'm willing to help with the review