Make manual-down the default for akka-cluster

Since we expect minimal partitioning and the we expect a restarted node to always rejoin a cluster and because we do not expect to add new nodes to the cluster in the near term it makes sense to make manual-downing the default model instead of using auto-down.

It is possible to manual-down a node using the akka.cluster JMX beans. It should therefore be possible to write a script which uses something like jolokia to manually down a node when necessary.