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?
Only issue with running this under a kubelet is that the salt default binds the apiserver to `127.0.0.1` (https://github.com/GoogleCloudPlatform/kubernetes/blob/master/cluster/saltbase/salt/apiserver/default#L13). Based on the current salt config, there isn't a way to communicate from a container running on a kubelet to the apiserver on the master. 

Are okay with binding the apiserver to a real IP? If so, we still need a way to pass that information into nginx. It looks like a pre-nginx start script would be required to read a ENV andsed the .conf files.