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. Per-node controller _is_ using the scheduler interface in this case.  But I don't think we want a single Pod object to represent creating a pod on a set of nodes.  Lots of things will break (e.g. ```kubectl logs ...```)

2. I don't think we want to achieve this with constraints, it's a backwards way into the solution we want.

3.  Yes, I'm open to that idea.  If we'd rather move Host into Spec we can achieve the same thing as this PR, but I think that we may want to still have general field selectors.