Drop *openstack/common* in flake8 exclude list
Drop *openstack/common* in flake8 exclude list

There is not this directory in debtcollector,so we should drop it
for improving searching efficiency.

Change-Id: Id06c62a9a421a0578bf31bfdc9fa9936b2d7de8b

openstack/murano-agent,openstack/murano-agent,openstack/murano-agent,openstack/murano-agent