Add support for Rabbit Clustering using a single IP address.

Previsouly, you could only cluster Rabbit Brokers
if you specified multiple rabbit_hosts.

This patch removes the requirement to have multiple Rabbit host addresses.
You can now front-end the Rabbit Brokers using a load-balancer to
use Rabbit Mirrored Queues (rabbit_ha_queues).

OpenStack services will connect to a single, Virtual IP address
for high-availability and scalability purposes.

Defaults to false for backwards compatability and to not
implement rabbit_ha_queues.

Change-Id: I6dec8e7ca8869c0a9e002c5252c9b3c29390306b