fixed behavior for rabbit_hosts variable

We identified an issue where if a user had previously defined rabbit_host,
then went to rabbit_hosts, the old value would cause confusion in many
of the OpenStack tools. This patch ensures that when rabbit_hosts is
defined, rabbit_host is absent, to avoid any configuration problems.

Change-Id: I8b5c497d2bb4f3e637aff0dd7cf61f91f173027c
Merge "fixed behavior for rabbit_hosts variable"