support global-requirements.txt

We are changing the upstream requirements.txt and
test-requirements.txt to a combinded global-requirements.txt.
Changes the checking script to support this as a possible file as
well. This should be a seemless backwards compatible change.

While modifying this file, also fix the flake8 style issues.

Change-Id: I95ccd6f721a2c7f4e873d1de171d569bae0631e8
Merge "support global-requirements.txt"