On reconnecting a FanoutConsumer, don't grow the topic name

Commit 719eba4cae7f2bc11753b17c65c7597b8d6ed72d in oslo-incubator
introduced a small bug on reconnect. Because of the way the topic
was extracted, an extra '_fanout' gets tacked onto the topic name
with each reconnect. A one line fix in code (rpartition) and test
case are needed.

Change-Id: I7b3ca48502c9d5a99802c24dfcec0a1a8292fa07
Fixes: bug #1201552
Merge "On reconnecting a FanoutConsumer, don't grow the topic name"