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).
> Once the "scheduler separation is completed", I imagine CreatePod will have a version that doesn't take a machine, 
Correct. This is just a shim so I can make progress without breaking things.

>It probably would not call assignpod at all 

Right.