Misc fixes after FailureDetectorPuppet and abstraction review

- Moved FailureDetectorPuppet to its own file in src/test.
- Removed 'phi' method from FailureDetector public API.
- Throwing exception instead of falling back to default if we can't load the custom FD.
- Removed add-connection method in FailureDetectorPuppet.

Signed-off-by: Jonas Bonér <jonas@jonasboner.com>

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

@@ -67,7 +67,7 @@ class ClusterSpec extends AkkaSpec(ClusterSpec.config) with BeforeAndAfter {
     @volatile
     var _unavailable: Set[Address] = Set.empty
 
-    override val failureDetector = new AccrualFailureDetector(system, clusterSettings) {
+    override val failureDetector = new FailureDetectorPuppet(system) {
       override def isAvailable(connection: Address): Boolean = {
         if (_unavailable.contains(connection)) false
         else super.isAvailable(connection)