Provide a lightweight garbd role as an alternative for the 3rd controller node

The required amount of HA controller nodes for production ready clusters is >=3.
Fuel should provide a deployment alternative for 3rd controller node to be decomposed as the set of following roles:
* DB: Garbd arbitrator role should be added as a lightweight galera node in cluster. This role should be allowed to be combined with any other existing roles. This node should not process any DB requests, the only purpose is to preserve the quorum and prevent the split brain.
* AMQP: RabbitMQ cluster of 2 nodes with mirrored queues should be able to run as is w/o issues (so, no changes here)
* Corosync and Pacemaker: then Garbd role assigned to some node, whether it has another roles assigned or not, Fuel must provide the node's membership in the Corosync & Pacemaker cluster as well.
* Swift: replication factor should be lowered to 1 so the two-nodes swift cluster should operate as is.
* Ceph/Cinder storage: not affected, these roles are good to go with any nodes
* Mongo: TBD (the impact to be elaborated)
* Zabbix: TBD (the impact to be elaborated)

Alternative: make galera as a separate deployable role and install it on any node as a lightweight arbitrator, which is a galera node participating only in quorum membership activities and which cannot
process SQL connections.