Support Docker and ContainerInfo

* Bump the mesos dependency to 0.20.0
* Create ExternalContainerInfo and ContainerInfo classes in
  MesosSlaveInfo to better map with the Mesos API.
    - ExternalContainerInfo replaces can be used by external
      containerizers
    - ContainerInfo is to be used with the new containerizer system
      (only DOCKER is supported right now)
* Add ability to specify Volumes
* Refactor a bit the config parts to use the MesosCloud and
  MesosSlaveInfo objects everywhere.