Tighten flake8 checks in Gating

There are a few checks that already pass:
 - F401 - unused imports of modules
 - F403 - ‘from module import *’ used; unable to detect undefined names

And there are two (H401 and H403) that can be made
passing easily. Enable gating on all of them.

Change-Id: I9d3c49ecea1b2171d194ab6cc751316c252081e8
Merge "Tighten flake8 checks in Gating"