Use join_consumer_pool() for notifications

Instead of using a private API only available to the kombu
driver, use the new public API for receiving messages
like notifications.

blueprint move-listener-framework-oslo
bug 1047015
bug 1111632

Change-Id: Ibf4c0fe7efca973f88b2eb880c920f2399de69d8
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Merge "Use join_consumer_pool() for notifications"