Remove *openstack/common* from flake8 excclude list in tox.ini
Remove *openstack/common* from flake8 excclude list in tox.ini

The *openstack/common* was used to keep codes from oslo-incubator,
but oslo-incubator was ignore in oslo3.11.0. So I think we
needn't use it any more.
Here is the oslo.utils CHANGES link:
http://docs.openstack.org/developer/oslo.utils/history.html

Change-Id: Ia8f46f8192bed49a7a7d71103c45a6ca5d2f6012

openstack/tacker,zeinsteinz/tacker,openstack/tacker,openstack/tacker,stackforge/tacker,stackforge/tacker,zeinsteinz/tacker