Don't include openstack/common in flake8 exclude list
Don't include openstack/common in flake8 exclude list

There is no this directory openstack/common which was used to keep codes
from oslo-incubator, we have retired oslo-incubator. Removing
openstack/commonfrom all OpenStack code in favor of the Oslo libraries
is a project wide goal for the Ocata release. So don't use this
directory any more. We should drop it for improving searching
efficiency.

Change-Id: Ib0c00251d72eadea997aba90e8c906d022745558

openstack/openstack-user-stories