Cluster node roles, see #3049

* Config of node roles cluster.role
* Cluster router configurable with use-role
* RoleLeaderChanged event
* Cluster singleton per role
* Cluster only starts once all required per-role node
  counts are reached,
  role.<role-name>.min-nr-of-members config
*  Update documentation and make use of the roles in the examples
Merge pull request #1254 from akka/wip-3049-cluster-roles-patriknw

Cluster node roles, see #3049