Fix leaking this in constructor of Cluster, see #2473

* Major refactoring to remove the need to use special
  Cluster instance for testing. Use default Cluster
  extension instead. Most of it is trivial changes.
* Used failure-detector.implementation-class from config
  to swap to Puppet
* Removed FailureDetectorStrategy, since it doesn't add any value
* Added Cluster.joinSeedNodes to be able to test seedNodes when Addresses
  are unknown before startup time.
* Removed ClusterEnvironment that was passed around among the actors,
  instead they use the ordinary Cluster extension.
* Overall much cleaner design

akka-cluster/src/main/scala/akka/cluster/ClusterEvent.scala

@@ -169,7 +169,7 @@ object ClusterEvent {
  * Responsible for domain event subscriptions and publishing of
  * domain events to event bus.
  */
-private[cluster] final class ClusterDomainEventPublisher(environment: ClusterEnvironment) extends Actor with ActorLogging {
+private[cluster] final class ClusterDomainEventPublisher extends Actor with ActorLogging {
   import InternalClusterAction._
 
   var latestGossip: Gossip = Gossip()