[minigraph][port_config] Use imported config.main and add conditional patch

[Blueve]

What I did

We found an issue where UT couldn't pass in old release branch with patch #1705

How I did it

To solve this issue, I applied two change in this PR:

• Use imported config.main module since we are not import config.main as config in old release branches
• Add conditional patch since some file were not mocked in old release branches

How to verify it

• Test on master
• Test on 202012 [minigraph][port_config] Use imported config.main and add conditional patch #1728
• Test on 201911 [minigraph][port_config] Consume port_config.json while reloading minigraph #1725
• Test on 201811 [minigraph][port_config] Consume port_config.json while reloading minigraph #1726
• Test on 202106

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)