enable_rabbitmq_cluster is now a "yes" by default but you can set it
to "no" if you want to disable clustering under any circumstances.

The agreement made at OpenStack in Austin was that Kolla-Kubernetes
would concentrate on RabbitMQ and MariaDB without clustering but
with persistent storage and workload migration, then examine how to
do proper distributed functionality as the project progresses, so I
am just following what we'd already agreed upon.

First, it helps us deal with issues of version upgrades without
dealing with clustered version upgrades and the synchronization
thereof.

Second, it provides an alternative model for durability when used in
Kubernetes.  Understand that, if we disable RabbitMQ's clustering,
Kubernetes is still able to re-schedule the queue off of a failed node
in ways that Kolla-Ansible is not.  There are known issues with
RabbitMQ clustering, especially with auto-heal turned on.  For many
small-to-mid-sized clusters, it's going to provide for a better
operator experience to have the known potential for a 30 second blip
after RabbitMQ node failure than it is to have the known potential
for partition and data loss and/or manual operations after you've
turned off auto-heal.

Kolla-kubernetes has already turned off host networking for the
RabbitMQ pod; it's safe to set the interface address in the
Kubernetes context.

The question was asked why don't I just set the RabbitMQ cluster to be
a single instance.  It's unlikely that Kubernetes RabbitMQ with a
PetSet will be clustered in the same declaritive fashion as the
rabbitmq-clusterer plugin. Easier to just disable it and worry about
how to configure the kube-friendly clustered RabbitMQ at a later point
in time.  Furthermore, it's an entirely valid case for many OpenStack
control planes hosted atop Kolla-Kubernetes to accept the possibility
of a 30-60 second blip in lieu of the long and questionable history
of RabbitMQ clustering in production.

Co-authored-by: Ryan Hallisey <rhallise@redhat.com>
Change-Id: I7f0cb22d29a418fce4af8d69f63739859173d746
Partially-implements: blueprint api-interface-bind-address-override