[SPARK-5965] Standalone Worker UI displays {{USER_JAR}}

For screenshot see: https://issues.apache.org/jira/browse/SPARK-5965
This was caused by 20a6013106.

Author: Andrew Or <andrew@databricks.com>

Closes #4739 from andrewor14/user-jar-blocker and squashes the following commits:

23c4a9e [Andrew Or] Use right argument

core/src/main/scala/org/apache/spark/deploy/worker/ui/WorkerPage.scala

@@ -134,7 +134,7 @@ private[spark] class WorkerPage(parent: WorkerWebUI) extends WebUIPage("") {
   def driverRow(driver: DriverRunner): Seq[Node] = {
     <tr>
       <td>{driver.driverId}</td>
-      <td>{driver.driverDesc.command.arguments(1)}</td>
+      <td>{driver.driverDesc.command.arguments(2)}</td>
       <td>{driver.finalState.getOrElse(DriverState.RUNNING)}</td>
       <td sorttable_customkey={driver.driverDesc.cores.toString}>
         {driver.driverDesc.cores.toString}