[SPARK-6537] UIWorkloadGenerator: The main thread should not stop SparkContext...

[SPARK-6537] UIWorkloadGenerator: The main thread should not stop SparkContext until all jobs finish

The main thread of UIWorkloadGenerator spawn sub threads to launch jobs but the main thread stop SparkContext without waiting for finishing those threads.

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

Closes #5187 from sarutak/SPARK-6537 and squashes the following commits:

4e9307a [Kousuke Saruta] Fixed UIWorkloadGenerator so that the main thread stop SparkContext after all jobs finish