Reintroduce 'seed' nodes, see #2219

* Implement the join to seed nodes process
  When a new node is started started it sends a message to all
  seed nodes and then sends join command to the one that answers
  first.
* Configuration of seed-nodes and auto-join
* New JoinSeedNodeSpec that verifies the auto join to seed nodes
* In tests seed nodes are configured by overriding seedNodes
  function, since addresses are not known before start
* Deputy nodes are the live members of the seed nodes (not sure if
  that will be the final solution, see ticket 2252
* Updated cluster.rst with latest info about deputy and seed nodes

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

@@ -0,0 +1,46 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.cluster
+
+import com.typesafe.config.ConfigFactory
+import org.scalatest.BeforeAndAfter
+import akka.remote.testkit.MultiNodeConfig
+import akka.remote.testkit.MultiNodeSpec
+import akka.testkit._
+import akka.util.duration._
+
+object JoinSeedNodeMultiJvmSpec extends MultiNodeConfig {
+  val seed1 = role("seed1")
+  val seed2 = role("seed2")
+  val ordinary1 = role("ordinary1")
+  val ordinary2 = role("ordinary2")
+
+  commonConfig(debugConfig(on = false).withFallback(MultiNodeClusterSpec.clusterConfig))
+}
+
+class JoinSeedNodeMultiJvmNode1 extends JoinSeedNodeSpec with FailureDetectorPuppetStrategy
+class JoinSeedNodeMultiJvmNode2 extends JoinSeedNodeSpec with FailureDetectorPuppetStrategy
+class JoinSeedNodeMultiJvmNode3 extends JoinSeedNodeSpec with FailureDetectorPuppetStrategy
+class JoinSeedNodeMultiJvmNode4 extends JoinSeedNodeSpec with FailureDetectorPuppetStrategy
+
+abstract class JoinSeedNodeSpec
+  extends MultiNodeSpec(JoinSeedNodeMultiJvmSpec)
+  with MultiNodeClusterSpec {
+
+  import JoinSeedNodeMultiJvmSpec._
+
+  override def seedNodes = IndexedSeq(seed1, seed2)
+
+  "A cluster with configured seed nodes" must {
+    "join the seed nodes at startup" taggedAs LongRunningTest in {
+
+      startClusterNode()
+      enterBarrier("all-started")
+
+      awaitUpConvergence(4)
+
+      enterBarrier("after")
+    }
+  }
+}