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).
Is Infeasible a commonly used term in scheduling?  The condition feels more like "GaveUp" or "CouldNotSchedule" - I just anticipate explaining Infeasible to users of naive clients in the future, was hoping for something more related to scheduling.  "OutOfSpace"?  "NotPossible"?