zmq tests should reset config between tests

In test_zmq.py setUp() we do FLAGS.set_override(), but we don't have a corresponding FLAGS.reset() in tearDown()

This means that that running a test alters the state of FLAGS for all other tests. We should ensure a clean slate after each test.

You can see the subtleties this introduces here:

        ipc_dir = FLAGS.rpc_zmq_ipc_dir

        internal_ipc_dir = '/tmp/openstack-zmq.ipc.test'

        if not os.path.exists(os.path.join(ipc_dir, "zmq_topic_zmq_replies")):
            FLAGS.set_default('rpc_zmq_ipc_dir', internal_ipc_dir)
            ....
        elif ipc_dir != internal_ipc_dir:
            ....

The elif is specifically for the case where FLAGS.rpc_zmq_dir was not overwritten by a previous test.