Refactor internal API for Pods to match v1beta3

Users look at Host. The scheduler uses Host to determine which Pods haven't yet been scheduled, or used to. 

Somehow, the host chosen by the scheduler needs to be reported.

The scheduler creates a Binding, which in turn causes a BoundPod to be created. BoundPods are indexed by host, though. If we kept all the objects cached in memory, this wouldn't be hard to repopulate from BoundPods.

Given that we don't do that, the easiest thing would be to ensure that Host is stored in etcd. We'll want to store other status info in etcd, such as termination info and events, also, treating it as a cache of information collected from nodes.

Without this line, how is Host initialized?

If this is too hard to fix at this point, I'd be fine with putting another copy of Host in api.PodSpec, and comment that it's deprecated and should be removed once we figure out a solution to this.