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 in following patch
https://review.openstack.org/#/c/297487/
so don't use this directory any more.

Change-Id: Id20b9cb8cd63e018a2f566cb12600b8f9e518665

noironetworks/heat,noironetworks/heat,openstack/heat,openstack/heat