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 - what should a client like ReplicationController do when it gets Infeasible?  Delete the pod and try again?  Backoff?  Or should there be something that periodically rechecks Infeasible (seems expensive and a waste of resources in some cases).  Infeasible *seems* like it's the pathological case - you will never get satisfied.  NoMoreResources seems like a temporary thing (after all, if I go delete X, Y, and Z, expecting the pod to get rescheduled, what do I do?).