[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.

Author: WangTao <barneystinson@aliyun.com>
Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #3266 from WangTaoTheTonic/killsubmit and squashes the following commits:

e03eba5 [WangTaoTheTonic] add comments
57b5ca1 [WangTao] SparkSubmitDriverBootstrapper should stop after its SparkSubmit sub-process ends