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).
>Abusive scheduling (schedule X thousand of something) seems like something the apiserver has to defend against...

That's a good point. Apiserver should defend against this, not a scheduler. OK. So api server has to do a quota check. :( :(