Prepare for external scheduler

1. Change name of pod statuses (Waiting, Running, Terminated)
2. Store assigned host in etcd.
3. Change pod key to /registry/pods/<podid>. Container location remains
   the same (/registry/hosts/<machine>/kublet).
Per what @bgrant0607 was sayaing, we'll disambiguate those cases with a StatusReason/StatusDetails field, to be added in another PR.