Set rabbit_hosts to a list of controllers for Neutron

It's addition to the https://review.openstack.org/#/c/77409
This commit shouldn't be merged before https://review.openstack.org/#/c/77409
It also can fail tests without it.

Change-Id: I3f816ce9bb98b42b80794c28fd9fc21e55e4084b
Partial-bug: #1285449