Specify hostpath volume and mount the volume in Spark driver and executor pods

[sumansomasundar]

In some cases (for example, log aggregation) it is desirable to have a hostpath volume mounted in the driver and executor pods. With #38 this will be possible, but I am not sure when it will be available.

So, something similar to #397 will be helpful for this case as well.

What I am proposing is an option to specify the hostpath volume and have it mounted in the driver and executor pods.

[sumansomasundar]

I have made changes that support the following configuration for spark-submit command:
--conf spark.kubernetes.driver.hostpath.<hostpath_dir>=<container_mount_path>
If this approach sounds good, I can create a pull request for this change.