Trivial: remove openstack/common in flake8 exclude list
Trivial: remove openstack/common in flake8 exclude list

There is no directory openstack/common any more, so ignore
it.

Change-Id: Ia1bb265a34d64be2d3cb31dbf6b0de42e90ec68a

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar