Continued work on rabbit host methods

Did not increase the version, b/c 1.3.0 is still under development.
We should not migrate to this version, until we complete this work.
We are using the new ["openstack"]["mq"] attributes.  It is up
to the operator to install rabbit, and make these ["openstack"]["mq"]
attributes available.
* The #rabbit_servers method will return the ["openstack"]["mq"]["servers"]
  value if defined.  Otherwise, it will perform a search, and construct
  a comma-delimited list of servers.
* The #rabbit_server method will return the ["openstack"]["mq"]["server"]
  value if defined.  Otherwise, will perform a search, and return the first
  rabbit server found.

Change-Id: Ia3771d70ea7eeb367b1fa17c5258f719528b5d22