Prepare for external scheduler

1. Change name of pod statuses (Waiting, Running, Terminated)
2. Store assigned host in etcd.
3. Change pod key to /registry/pods/<podid>. Container location remains
   the same (/registry/hosts/<machine>/kublet).
Sure. I was just pointing out that the comment (and name "Stopped") isn't 100% accurate, since it might never have been "Running", or it may in fact still be running, but in a disconnected mode. Not sure I have a better name that encompasses all terminal states. In Mesos, we use "Lost" to indicate alternative non-running states.