Add an implicit node name label for scheduling pods.

This enables easy scheduling to specific nodes, which is a requirement for a "pod per node" controller.

See #1518 
I meant how would the pod interface change. It shouldn't be possible to specify `$.Name` in a NodeSelector. If it is, that's a validation bug.

Options:
1. Per-node controller can use the scheduler interface. There would need to be a way to specify which "scheduler" would be responsible for scheduling.
2. #367 discusses constraints along the lines of per-attribute limits.
3. Enable the host to be set upon pod creation. The host fields are currently in Status in v1beta3. We could move them back to the Spec. When we split out the pod template (#170), we'd want to create a way of overriding the host field when creating a pod from a template.

IMO, all of these are things we want eventually for other reasons. However, the advantage of (2) for this use case is that it would make the per-node controller more similar to ReplicationController, probably similar enough that it would make sense to unify them.