Refactor cluster startup join in tests and fix barrier race

* Refactored common code to MultiNodeClusterSpec.awaitClusterUp
* Fixed some race conditions of barriers

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

@@ -34,13 +34,10 @@ class ClientDowningNodeThatIsUnreachableSpec
   "Client of a 4 node cluster" must {
 
     "be able to DOWN a node that is UNREACHABLE (killed)" taggedAs LongRunningTest in {
+      val thirdAddress = node(third).address
+      awaitClusterUp(first, second, third, fourth)
+
       runOn(first) {
-        startClusterNode()
-        awaitUpConvergence(numberOfMembers = 4)
-
-        val thirdAddress = node(third).address
-        testConductor.enter("all-up")
-
         // kill 'third' node
         testConductor.shutdown(third, 0)
 
@@ -50,28 +47,19 @@ class ClientDowningNodeThatIsUnreachableSpec
 
         awaitUpConvergence(numberOfMembers = 3, canNotBePartOfMemberRing = Seq(thirdAddress))
         cluster.latestGossip.members.exists(_.address == thirdAddress) must be(false)
-        testConductor.enter("await-completion")
       }
 
       runOn(third) {
-        cluster.join(node(first).address)
-
-        awaitUpConvergence(numberOfMembers = 4)
-        testConductor.enter("all-up")
+        testConductor.enter("down-third-node")
       }
 
       runOn(second, fourth) {
-        cluster.join(node(first).address)
-        awaitUpConvergence(numberOfMembers = 4)
-
-        val thirdAddress = node(third).address
-        testConductor.enter("all-up")
-
         testConductor.enter("down-third-node")
 
         awaitUpConvergence(numberOfMembers = 3, canNotBePartOfMemberRing = Seq(thirdAddress))
-        testConductor.enter("await-completion")
       }
+
+      testConductor.enter("await-completion")
     }
   }
 }