[SPARK-5329][WebUI] UIWorkloadGenerator should stop SparkContext.

UIWorkloadGenerator don't stop SparkContext. I ran UIWorkloadGenerator and try to watch the result at WebUI but Jobs are marked as finished.
It's because SparkContext is not stopped.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #4112 from sarutak/SPARK-5329 and squashes the following commits:

bcc0fa9 [Kousuke Saruta] Disabled scalastyle for a bock comment
86a3b95 [Kousuke Saruta] Fixed UIWorkloadGenerator to stop SparkContext in it

core/src/main/scala/org/apache/spark/ui/UIWorkloadGenerator.scala

@@ -22,20 +22,23 @@ import scala.util.Random
 import org.apache.spark.{SparkConf, SparkContext}
 import org.apache.spark.scheduler.SchedulingMode
 
+// scalastyle:off
 /**
  * Continuously generates jobs that expose various features of the WebUI (internal testing tool).
  *
- * Usage: ./bin/spark-class org.apache.spark.ui.UIWorkloadGenerator [master] [FIFO|FAIR]
+ * Usage: ./bin/spark-class org.apache.spark.ui.UIWorkloadGenerator [master] [FIFO|FAIR] [#job set (4 jobs per set)]
  */
+// scalastyle:on
 private[spark] object UIWorkloadGenerator {
 
   val NUM_PARTITIONS = 100
   val INTER_JOB_WAIT_MS = 5000
 
   def main(args: Array[String]) {
-    if (args.length < 2) {
+    if (args.length < 3) {
       println(
-        "usage: ./bin/spark-class org.apache.spark.ui.UIWorkloadGenerator [master] [FIFO|FAIR]")
+        "usage: ./bin/spark-class org.apache.spark.ui.UIWorkloadGenerator " +
+          "[master] [FIFO|FAIR] [#job set (4 jobs per set)]")
       System.exit(1)
     }
 
@@ -45,6 +48,7 @@ private[spark] object UIWorkloadGenerator {
     if (schedulingMode == SchedulingMode.FAIR) {
       conf.set("spark.scheduler.mode", "FAIR")
     }
+    val nJobSet = args(2).toInt
     val sc = new SparkContext(conf)
 
     def setProperties(s: String) = {
@@ -84,7 +88,7 @@ private[spark] object UIWorkloadGenerator {
       ("Job with delays", baseData.map(x => Thread.sleep(100)).count)
     )
 
-    while (true) {
+    (1 to nJobSet).foreach { _ =>
       for ((desc, job) <- jobs) {
         new Thread {
           override def run() {
@@ -101,5 +105,6 @@ private[spark] object UIWorkloadGenerator {
         Thread.sleep(INTER_JOB_WAIT_MS)
       }
     }
+    sc.stop()
   }
 }