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: I828fbf5b1ee1435a85ffe5eb2295064b36064295

stackforge/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient,openstack/python-senlinclient