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 
1. By "scheduler interface", I meant the per-node controller could POST `Binding`s, just as the scheduler does, and I definitely was not suggesting a single Pod would represent multiple pods.
2. I agree that per-attribute limits would be less explicit, but OTOH, it could extend to use cases such as one per rack, which we've needed internally for power and network monitoring and liveness/reachability-assessment agents, as well as for more explicit control over fault tolerance.
3. If you want to support the selector approach, I'd use a label namespace rather than funky syntax. The current syntax in this PR should be rejected by the API.