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).
I did change it to Terminated, which is admittedly not much different. I view both words as a current state and not carrying the implication that it previously was in a Running state. E.g., a pod could go directly from waiting to terminated under some unfortunate circumstances. "Lost" sounds like it's always a bad thing to me, whereas "Terminated" sounds like it could normal or bad depending on exactly why.