Consider using GET to deploy the Gitaly demo environments

The Gitaly team regularly run demos using environments set up via Terraform and Ansible.

This seems like an ideal job for the GET, particularly if it's intended to centralize that kind of tooling.

This issue is for the GET and Gitaly teams to consider if there's value in the proposal, and if so, what needs to be done.

Tasks

• Educate Gitaly people how to use GET
• Make sure they have proper access to a GCP project to run GET on
• Replace demo scripts with docs pointing to here and how to use this for demos