[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