Change cleanup behaviour in test_pub_sub.py
Change cleanup behaviour in test_pub_sub.py

Changed the subscriber to connect to the loopback - as the publisher is
bound on that address, instead of the local host IP.
Modified cleanup to use addCleanup methods instead of manually
cleaning-up. This is because if an assertion fails before the cleanup
today, it will not clean-up and the next tests may fail.

Change-Id: I290d4b1e5378831e963d2f48f8e5a9fa8b17cd1a

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow