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.
With this configuration, all Kubernetes logs within the ATAT cluster
will be sent to AWS CloudWatch.
Note that this requires applying an additional IAM policy to the worker
nodes' role.
