remove openstack/common from the exclude list of flake in tox.ini
remove openstack/common from the exclude list of flake in tox.ini

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: I38abf7f3da42b36648535c2b63167a089c186e24

openstack/ceilometer-powervm,stackforge/ceilometer-powervm,openstack/ceilometer-powervm,stackforge/ceilometer-powervm