[NOT FOR MERGE] Add option to force docker pull

_Do not merge yet.  This patch depends on a not-yet-released version of Mesos (0.22.0)._

Fixes ##837.

TODO:

- [ ] Release a `0.22.0` version of mesos-utils.
- [ ] Update the mesos-utils dependency version in the Marathon build definition.
- [ ] Update documentation to describe the new parameter.