[SPARK-9497] [SPARK-9509] [CORE] Use ask instead of askWithRetry

`RpcEndpointRef.askWithRetry` throws `SparkException` rather than `TimeoutException`. Use ask to replace it because we don't need to retry here.

Author: zsxwing <zsxwing@gmail.com>

Closes #7824 from zsxwing/SPARK-9497 and squashes the following commits:

7bfc2b4 [zsxwing] Use ask instead of askWithRetry

core/src/main/scala/org/apache/spark/deploy/client/AppClient.scala

@@ -27,7 +27,7 @@ import org.apache.spark.deploy.{ApplicationDescription, ExecutorState}
 import org.apache.spark.deploy.DeployMessages._
 import org.apache.spark.deploy.master.Master
 import org.apache.spark.rpc._
-import org.apache.spark.util.{ThreadUtils, Utils}
+import org.apache.spark.util.{RpcUtils, ThreadUtils, Utils}
 
 /**
  * Interface allowing applications to speak with a Spark deploy cluster. Takes a master URL,
@@ -248,7 +248,8 @@ private[spark] class AppClient(
   def stop() {
     if (endpoint != null) {
       try {
-        endpoint.askWithRetry[Boolean](StopAppClient)
+        val timeout = RpcUtils.askRpcTimeout(conf)
+        timeout.awaitResult(endpoint.ask[Boolean](StopAppClient))
       } catch {
         case e: TimeoutException =>
           logInfo("Stop request to Master timed out; it may already be shut down.")