Support running Docker containers

Mesos 0.20 will be released soon, and it will likely include support for a docker containerizer. See MESOS-1524.

To make use of this feature, I propose modifying aurora's DSL to support running docker containers both with and without the aurora executor.

Task would be changed to introduce a container field and make processes optional when a container is specified. A Task to launch a Docker container using the entry point (aka command) specified in the container would look like this:

Task(
    name="my-task",
    container=Docker(
        image="docker:///centos:6u5',
    ),
    resources=Resources(cpu=1, disk=10*GB, ram=1*GB)
)

If you specify processes in your Task then the docker containerizer will fetch and launch the aurora executor for you. This would run process_a then process_b inside the same docker container using the aurora executor:

Task(
    name="my-task",
    container=Docker(
        image="docker:///centos:6u5',
    ),
    processes=[process_a, process_b],
    constraints=order(process_a, process_b),
    resources=Resources(cpu=1, disk=10*GB, ram=1*GB)
)

The Docker() struct would closely model the fields that will be in the DockerInfo protobuf message detailed here: https://github.com/tnachen/mesos/wiki/DockerInfo-design