Migrate default-http-backend manifests into ingress-controller dir

The default-http-backend is a required part of the ingress controller for successful function, so it's reasonable to colocate them.

A PR that moves the kubernetes/default-http-backend directory manifests into the nginx-ingress-controller directory and wires them into Kustomize would be most welcome.

I create #129 for this -- let me know if anything else needs to be done for this.