[SPARK-6059][Yarn] Add volatile to ApplicationMaster's reporterThread and allocator

`ApplicationMaster.reporterThread` and `ApplicationMaster.allocator` are accessed in multiple threads, so they should be marked as `volatile`.

Author: zsxwing <zsxwing@gmail.com>

Closes #4814 from zsxwing/SPARK-6059 and squashes the following commits:

17d9386 [zsxwing] Add volatile to ApplicationMaster's reporterThread and allocator

yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -68,8 +68,8 @@ private[spark] class ApplicationMaster(
   @volatile private var finalMsg: String = ""
   @volatile private var userClassThread: Thread = _
 
-  private var reporterThread: Thread = _
-  private var allocator: YarnAllocator = _
+  @volatile private var reporterThread: Thread = _
+  @volatile private var allocator: YarnAllocator = _
 
   // Fields used in client mode.
   private var actorSystem: ActorSystem = null