[SPARK-12396][CORE] Modify the function scheduleAtFixedRate to schedule.

Instead of just cancel the registrationRetryTimer to avoid driver retry connect to master, change the function to schedule.
It is no need to register to master iteratively.

Author: echo2mei <534384876@qq.com>

Closes #10447 from echoTomei/master.

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

@@ -124,7 +124,7 @@ private[spark] class AppClient(
      */
     private def registerWithMaster(nthRetry: Int) {
       registerMasterFutures.set(tryRegisterAllMasters())
-      registrationRetryTimer.set(registrationRetryThread.scheduleAtFixedRate(new Runnable {
+      registrationRetryTimer.set(registrationRetryThread.schedule(new Runnable {
         override def run(): Unit = {
           Utils.tryOrExit {
             if (registered.get) {
@@ -138,7 +138,7 @@ private[spark] class AppClient(
             }
           }
         }
-      }, REGISTRATION_TIMEOUT_SECONDS, REGISTRATION_TIMEOUT_SECONDS, TimeUnit.SECONDS))
+      }, REGISTRATION_TIMEOUT_SECONDS, TimeUnit.SECONDS))
     }
 
     /**