min-nr-of-members and registerOnMemberUp, see #2306

* Leader moves joining members to up when min-nr-of-members reached
* Tested by MinMembersBeforeUpSpec
* Used in factorial sample
* Docs

akka-cluster/src/test/scala/akka/cluster/ClusterConfigSpec.scala

@@ -36,6 +36,7 @@ class ClusterConfigSpec extends AkkaSpec {
       JoinTimeout must be(60 seconds)
       AutoJoin must be(true)
       AutoDown must be(false)
+      MinNrOfMembers must be(1)
       JmxEnabled must be(true)
       UseDispatcher must be(Dispatchers.DefaultDispatcherId)
       GossipDifferentViewProbability must be(0.8 plusOrMinus 0.0001)