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: I7fd53fa9f3d5f6f8fa085bb103feea9b21baa16a