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 
But, as the discussion in #1518 implies, we don't even need to constrain each pod to a specific host. We just need to ensure at most one can schedule on each host.

That said, I think making it easy and efficient to request scheduling on a specific host is a useful escape hatch for all kinds of things.