Add contrib to tox pep8 check

This implements Clint's comment in
https://review.openstack.org/#/c/48690/1 adding # noqa
to the relative imports and adding contrib to the pep8 check.

run_tests.sh -p now returns no warnings.

Change-Id: I05085151258a6d0f3cc348211f64533fd4cbded9
Closes-Bug: #1234938
Merge "Add contrib to tox pep8 check"