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).
Actually, I guess specifically the quota check--if it's really as simple as comparing a pod's request against a user/group's assigned quota--I can see maybe being persuaded that that isn't really a scheduling problem and we could conceivably write a package to do that math, which could be used by apiserver before accepting the pod.

I still kinda feel like the scheduler should be allowed to reject the pods that it can with a cheap "does it fit" check.

I guess I'm also skeptical that the quota check is *really* that simple.