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.
This will allow Kubernetes resources that only pull images occasionally
(i.e., k8s jobs) to point to a static tag name, "latest", that is updated
regularly. It also means we can refer to that image in the k8s config
tracked in the repo, instead of out-of-date images.