A simpler ActorRefProvider config #20649 (#20767)

* Provide shorter aliases for the ActorRefProviders #20649
* Use the new actorefprovider aliases throughout code and docs
* Cleaner alias replacement logic

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

@@ -34,7 +34,7 @@ object MultiNodeClusterSpec {
     if (failureDetectorPuppet) clusterConfigWithFailureDetectorPuppet else clusterConfig
 
   def clusterConfig: Config = ConfigFactory.parseString("""
-    akka.actor.provider = akka.cluster.ClusterActorRefProvider
+    akka.actor.provider = cluster
     akka.cluster {
       jmx.enabled                         = off
       gossip-interval                     = 200 ms

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

@@ -124,7 +124,7 @@ private[cluster] object StressMultiJvmSpec extends MultiNodeConfig {
 
     akka.actor.serialize-messages = off
     akka.actor.serialize-creators = off
-    akka.actor.provider = akka.cluster.ClusterActorRefProvider
+    akka.actor.provider = cluster
     akka.cluster {
       failure-detector.acceptable-heartbeat-pause =  5s
       auto-down-unreachable-after = 1s

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

@@ -24,7 +24,7 @@ class SunnyWeatherMultiJvmSpec(artery: Boolean) extends MultiNodeConfig {
   val common =
     """
     akka {
-      actor.provider = akka.cluster.ClusterActorRefProvider
+      actor.provider = cluster
       loggers = ["akka.testkit.TestEventListener"]
       loglevel = INFO
       remote.log-remote-lifecycle-events = off