Prepare for external scheduler

1. Add more Pod statuses. Expectation that PodStatus will get changed by
   future scheduling actuator API, indicating acceptance/rejection of
   pod.
2. Store assigned host in etcd.
3. Change pod key to /registry/pods/<podid>. Container location remains
   the same (/registry/hosts/<machine>/kublet).
> Scheduler interface: +1 on not relying on shared state as an implicit API. I want an explicit API where the scheduler is notified of new pods and the scheduler can announce its assignment for each pod.

@adam-mesos Please also take a look at #592.