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/common from
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: Iec69bf3dedbff06ce9f7794b5569f35c5e88b970

openstack/networking-arista,stackforge/networking-arista,openstack/networking-arista