During the Train cycle, Bifrost switched to using JSON-RPC by default
for Ironic's internal communication [1], avoiding the need to install
RabbitMQ. This simplifies things, so we may as well remove our custom
configuration of RabbitMQ.

[1] https://review.openstack.org/645093

Change-Id: I3107349530aa753d68fd59baaf13eb7dd5485ae6