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

The directory openstack/common was used to keep codes from
oslo-incubator, we have retired oslo-incubator,so don't use
this directory any more.

Change-Id: I0750b2f3aa25b079ca9ab95d28353dfdce7a3428

openstack/barbican,openstack/barbican