engine: Introduce RabbitMQ broker type

RabbitMQ is a popular messaging broker which is
being supported by OpenStack. oVirt engine should
support configuring RabbitMQ in addition to Qpid,
so users can select their desired messaging broker.

Change-Id: Ic3fa551c1d416a53df47e940518c933722a80867
Bug-Url: https://bugzilla.redhat.com/1078862
Signed-off-by: Moti Asayag <masayag@redhat.com>