[SPARK-4404]SparkSubmitDriverBootstrapper should stop after its SparkSubmit sub-proc...

...ess ends

https://issues.apache.org/jira/browse/SPARK-4404

When we have spark.driver.extra* or spark.driver.memory in SPARK_SUBMIT_PROPERTIES_FILE, spark-class will use SparkSubmitDriverBootstrapper to launch driver.
If we get process id of SparkSubmitDriverBootstrapper and wanna kill it during its running, we expect its SparkSubmit sub-process stop also.
No need to name this thread. Also, could you add a comment here to explain that this is for killing the app while it's running? Otherwise it's not clear to me why we need to do this here and do the same at the end of the file again.