CORE-1544 Setting static rabbitmq override

This commit sets a manual override for rabbitmq as the default
system override is problematic and can result in states where
rabbitmq is not running but we are waiting for it to run.

Additionally, due to the override, this change tests for a
pre-existing cluster configuration, and if present, disables
the 40-rabbitmq configuration job.

Change-Id: Ia5adb1e3a32941d4e1f1c46abd4e68e9d17db6ed