Support --proxy-user for Spark on K8s

SPARK-23257 adds kerberized hdfs support for Spark on K8s. A major addition needed is the support for proxy user. A proxy user is impersonated by a superuser who executes operations on behalf of the proxy user. More on this: 

https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/Superusers.html

https://github.com/spark-notebook/spark-notebook/blob/master/docs/proxyuser_impersonation.md

This has been implemented for Yarn upstream and Spark on Mesos here:

https://github.com/mesosphere/spark/pull/26

ifilonenko creating this issue according to our discussion.