Kill drivers in postStop() for Worker.

 JIRA SPARK-1068:https://spark-project.atlassian.net/browse/SPARK-1068

Author: Qiuzhuang

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -325,6 +325,7 @@ private[spark] class Worker(
 
   override def postStop() {
     executors.values.foreach(_.kill())
+    drivers.values.foreach(_.kill())
     webUi.stop()
     metricsSystem.stop()
   }