Support for mesos DockerInfo in fine-grained mode.

This patch adds partial support for running spark on mesos inside of a docker container. Only fine-grained mode is presently supported, and there is no checking done to ensure that the version of libmesos is recent enough to have a DockerInfo structure in the protobuf (other than pinning a mesos version in the pom.xml).
mesos-0.21 doesn't appear up on maven central yet, not even an -rc release, so I'll add support for what is available in 0.20.1 for now, but leave it flexible enough to expand as new fields arrive.