[SPARK-15754][YARN] Not letting the credentials containing hdfs delegation tokens to be added in current user credential.

## What changes were proposed in this pull request?
The credentials are not added to the credentials of UserGroupInformation.getCurrentUser(). Further if the client has possibility to login using keytab then the updateDelegationToken thread is not started on client.

## How was this patch tested?
ran dev/run-tests

Author: Subroto Sanyal <ssanyal@datameer.com>

Closes apache#13499 from subrotosanyal/SPARK-15754-save-ugi-from-changing.

Author: Subroto Sanyal

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

@@ -980,7 +980,6 @@ private[spark] class Client(
     amContainer.setApplicationACLs(
       YarnSparkHadoopUtil.getApplicationAclsForYarn(securityManager).asJava)
     setupSecurityToken(amContainer)
-    UserGroupInformation.getCurrentUser().addCredentials(credentials)
 
     amContainer
   }
@@ -1001,7 +1000,8 @@ private[spark] class Client(
       sparkConf.set(KEYTAB.key, keytabFileName)
       sparkConf.set(PRINCIPAL.key, principal)
     }
-    credentials = UserGroupInformation.getCurrentUser.getCredentials
+    // Defensive copy of the credentials
+    credentials = new Credentials(UserGroupInformation.getCurrentUser.getCredentials)
   }
 
   /**