Run yang validation in db migrator

[ganglyu]

What I did

Add unit test/runtime alert for db_migrator.py/load_minigraph: ConfigDB passing yang validation

How I did it

Use subprocess to run yang validation, and then warm-reboot performance will not change. Check /etc/sonic/mgmt_test_mark, this mark_file means this device is under end to end test.

• For unit test, db_migrator will wait for validation result and raise exception.
• For end to end test, db_migrator will wait for validation result and raise exception.

How to verify it

Run unit test and run end to end test

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)