Prepare for external scheduler

1. Add more Pod statuses. Expectation that PodStatus will get changed by
   future scheduling actuator API, indicating acceptance/rejection of
   pod.
2. Store assigned host in etcd.
3. Change pod key to /registry/pods/<podid>. Container location remains
   the same (/registry/hosts/<machine>/kublet).
Also, quota is potentially an eventual consistency thing, and the limits may be soft and hard (discussed in another thread).  Abusive scheduling (schedule X thousand of something) seems like something the apiserver has to defend against (otherwise the scheduler gets more complex).  Soft limits could be enforced by a reconciliation style job.