Run flake8 also on cinder/*/openstack

exclude=openstack also matches cinder/api/openstack,
which is however not merged from oslo-incubator.
Exclude=common instead, and explicitely include
cinder/common

Change-Id: I331d8c0d17e519386ae72caeca8a336eecde3ad3
Merge "Run flake8 also on cinder/*/openstack"