MultiNodeSpec + Akka.Cluster bug fixes

Changes in this set:

* @smalldave has implemented most of the `MultiNodeConfig` and `MultiNodeSpec` - not totally done yet but most of the way there.
* Fixed a bug caused by new leaders being elected for a specific role after the cluster has already started.

I rebased on dev, so all of the commits relevant to this PR should be the 4 at the end.