Don't include openstack directory in exclude list for flake8
Don't include openstack directory in exclude list for flake8

Directory openstack doesn't exist, so don't need skip it.

Change-Id: I68c841019a52393bde5852bf390452c7aa127e94

masayukig/tempest,Juniper/tempest,sebrandon1/tempest,Tesora/tesora-tempest,openstack/tempest,Juniper/tempest,vedujoshi/tempest,cisco-openstack/tempest,masayukig/tempest,sebrandon1/tempest,openstack/tempest,cisco-openstack/tempest,vedujoshi/tempest,Tesora/tesora-tempest