[config] Enable YANG validation for override

[wen587]

What I did

Always enable YANG validation for load_minigraph during override. This is to fail early if golden config is invalid which will breaks GCU feature.

How I did it

Always enable YANG validation during load_minigraph with override

How to verify it

Unit 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)