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).
Because apiserver doesn't have any concept of quota or resources.

More specifically, scheduler needs to have that resource comparison logic anyway, and it seems really bad to me to have apiserver have a (possibly different) version of the same logic.

I think the scheduler is the only component that should be allowed to express an opinion about what can or can't fit in the cluster.