[SPARK-7373] [MESOS] Add docker support for launching drivers in mesos cluster mode.

Using the existing docker support for mesos, also enabling the mesos cluster mode scheduler to launch Spark drivers in docker images as well.

This also allows the executors launched by the drivers to be also in the same Docker image by passing  the docker settings.

Author: Timothy Chen <tnachen@gmail.com>

Closes #5917 from tnachen/spark_cluster_docker and squashes the following commits:

1e842f5 [Timothy Chen] Add docker support for launching drivers in mesos cluster mode.