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 
for 1) Yes, this is already the behavior, and that is the goal.  For example a label might be "ssd=true" and so I want to schedule onto any node that has SSD.

for 2) I don't think this will be a problem, because the host name label is always present, since it is programatically added by the scheduler, it's not added by the user.  So it can't be removed.