[SPARK-21585] Application Master marking application status as Failed for Client Mode

The fix deployed for SPARK-21541 resulted in the Application Master to set the final status of a spark application as Failed for the client mode as the flag 'registered' was not being set to true for client mode. So, in order to fix the issue, I have set the flag 'registered' as true in client mode on successfully registering Application Master.

## How was this patch tested?
Tested the patch manually.

Before:
<img width="1275" alt="screen shot-before2" src="https://user-images.githubusercontent.com/22228190/28799641-02b5ed78-760f-11e7-9eb0-bf8407dad0ad.png">

After:
<img width="1221" alt="screen shot-after2" src="https://user-images.githubusercontent.com/22228190/28799646-0ac9ef14-760f-11e7-8bf5-9dfd743d0f2f.png">

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: pgandhi <pgandhi@yahoo-inc.com>
Author: pgandhi999 <parthkgandhi9@gmail.com>

Closes #18788 from pgandhi999/SPARK-21585.

resource-managers/yarn/src/main/scala/org/apache/spark/deploy/yarn/ApplicationMaster.scala

@@ -474,6 +474,7 @@ private[spark] class ApplicationMaster(
     addAmIpFilter()
     registerAM(sparkConf, rpcEnv, driverRef, sparkConf.getOption("spark.driver.appUIAddress"),
       securityMgr)
+    registered = true
 
     // In client mode the actor will stop the reporter thread.
     reporterThread.join()