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).
Doesn't look like you're using the selector anymore, just getting a list of all pods in the registry. Shouldn't you loop through the pods returned by ExtractList and check selector.Matches(labels.Set(pod.Labels))?