Remove cluster formation configuration from rabbitmq.config. Cluster
formation is managed by the task files in the role, see
tasks/rabbitmq_cluster.yml for details. The entries in the config
template are unnecessary and I believe they may be causing clustering
failures when the clustering tasks run.
Remove testing for cluster configuration in rabbitmq.config.
Change-Id: Ia65da05a5c967965143e8167657688d6906ad1f0
cadd8ff635