Unit tests of Cluster, see 2163

* ClusterSpec
- Test gossiping rules for deputies and unreachable
- Fix strange/wrong probabilites for gossip to unreachable and deputy nodes
- Fix lost order of Members when using map (without .toSeq) on the members SortedSet

* MemberSpec
- Test equals, hashCode

* GossipSpec
- Test member merge by status prio
- Fix bug in member merge (groupBy was wrong)

akka-cluster/src/multi-jvm/scala/akka/cluster/MultiNodeClusterSpec.scala

@@ -20,6 +20,7 @@ object MultiNodeClusterSpec {
       leader-actions-interval           = 200 ms
       unreachable-nodes-reaper-interval = 200 ms
       periodic-tasks-initial-delay      = 300 ms
+      nr-of-deputy-nodes                = 2
     }
     akka.test {
       single-expect-default = 5 s