Adds a [kustomize](https://github.com/kubernetes-sigs/kustomize) overlay
for a new staging environment. Additionally, adds environment variables
in the place of certain pieces of information that need to be templated.
The K8s README ("deploy/README.md") has been updated to reflect the new
method for applying config.
This commit also removes the configuration for the AWS cluster and
references to AWS in the README.
