Document that Kops Bastion is not a Kubernetes Node

[osterman]

what

Customer:

was surprised, too, that kubectl get nodes returned fewer nodes than we have instnaces. Lists 4 nodes and 3 masters but not the bastion

why

the kubelet does not run on the bastion in kops; i don't know why they designed it like that, but you're right. the bastion is not a kubernetes node.