The Pika driver was intended to be a more stable and performant
replacement for the default rabbit driver. However due to lack of
both maintainers and compelling evidence that pika is superior to the
existing rabbit driver in either performance or stability it has been
deprecated for removal.
See:
http://lists.openstack.org/pipermail/openstack-dev/2017-May/116679.html
Change-Id: I98e0123edd3248be665325833283689fc3a897f7