Drop *openstack/common* in flake8 exclude list
Drop *openstack/common* in flake8 exclude list

We have not *openstack/common* directory in vitrage, so
we should drop it for improving searching efficiency.

Change-Id: I9d78c4caf50571ff8cadef87eef471cf1c945875

openstack/vitrage,openstack/vitrage,openstack/vitrage