Explicitly require rabbitmq service before setting HA policy

This is yet another case of class containment gone wrong.  The
mirrored queues exec requires ::rabbitmq, and the latter includes
::rabbitmq::service which declares the service.  Since ::rabbitmq does
not 'contain' ::rabbitmq:service, the dependency does not get applied
to the mirrored queues exec.  Thus, it is possible for the exec to run
before the service is started.