Delete openstack/common from the exclude list of flake8 in tox
Delete openstack/common from the exclude list of flake8 in tox

The directory openstack/common was used to keep codes from
oslo-incubator, we have retired oslo-incubator, so don't use
this directory any more.

Change-Id: I3fcbec9fcec6d36c3e8254a809de0ae0b02a4841

openstack/ceilometermiddleware