Update mesos dependency to 0.20.0
Support Docker and ContainerInfo.

* Create ExternalContainerInfo and ContainerInfo classes in
  MesosSlaveInfo to better map with the Mesos API.
    - ExternalContainerInfo 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.
* Use MESOS_SANDBOX environement variable
  Fallback to './' if not present. This should work for docker and
  non-docker situation to run the slave.jar file.