Cluster node roles, see #3049

This is only a proposal of the first pieces of the solution. For discussion (not ready for merge).

- [x] Config of node roles cluster.role
- [x] Cluster router configurable with use-role
- [x] RoleLeaderChanged event
- [ ] ClusterSingleton per role
- [ ] Cluster only starts once all required per-role node counts are reached
- [ ] Update documentation and make use of the roles in the examples