Adds support for RabbitMQ Clustering

Previsouly, the openstack module only supported a single
RabbitMQ Broker.  This change adds the rabbit_hosts and
cluster_disk_nodes parameters used by the core modules to
manage rabbit clustering.

Defaults to false to disable rabbit clustering and for
backwards compatibility.

Change-Id: Iaae1ce3ec8864f43365bcf600ae3fa5c374d9b17
Merge "Adds support for RabbitMQ Clustering"