WIP: add kubernetes_nginx to contrib/

Looking for comments on a path forward.

This is a small addition to contrib/ that will hold the standard k8s nginx service which proxies to the apiserver. This is a step towards dockerizing all the various services (https://github.com/GoogleCloudPlatform/kubernetes/issues/19).

Currently this image relies on a host volume which is requires to have the SSL certificates and htpasswd files in the correct locations. Leveraging host volumes has the following benefits:

1. Allows this image to be generic and hosted on hub.docker or a private registry
2. Allows each provider to determine how they want to generate certs and place them appropriately (salt, etc).
3. Allows users to easily place signed SSL certificates if needed.

The primary downside is the fact that we have to manage files on the host which must be in place prior to the container starting.

Thoughts?
> comfortable with running the https proxy as a docker container on the master

First, thanks for this PR, I've been wanting a standard "gateway" pod. But don't run it this way, it's an anti pattern. I specifically got the kubernetes-ro service in to enable the sort of thing you're trying to do. :)