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).
That's true. I think eventually what will need to change here is the reason that it's in the waiting state. Also, in the PR that reconciles this with the Binding PR, I'll need to check that the pod is indeed a candidate for scheduling (is in Waiting with a blank Host field.)