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, and CreatePod(machine, pod) could then just call CreatePod(pod) then AssignPod(podID, machine), if CreatePod(machine, pod) is even needed anymore. Is this accurate?