Refactor PodStatus to PodCondition in internal API for v1beta3

Change PodStatus (string) to PodCondition (string) to make space for new PodStatus (struct) in v1beta3 API.

Change the constant values for Pod statuses to match v1beta3 API.
Yes, I agree that this stuff really should be watchable. @brendandburns is completely against storing it in etcd, although I would be fine with that (our internal system stores the internal equivalent of this stuff in our internal equivalent of etcd, and that works fine).