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).
It probably would not call assignpod at all - the scheduler would create the binding post pod creation and then the binding would be picked up by the kubelet.  To allow a scheduler to launch the task instead of the kubelet might involve some information on the Binding that allows the mapping of pod into the minions pod list to be skipped.