Adding Docker support!

* Updated to Mesos 0.20.1 & Marathon 0.7.1
* Created name for mesos cluster
* Docker now runs as a daemon in slave container (via breerly/mesos image)
* Slaves must run as privileged (docker cli has far-reaching needs)
* Added MESOS_CONTAINERIZERS config for slaves
* Created 3 slaves with separate ports
* Locked zookeeper version