Add Aliases for Common GitLab Site Descriptions

Use Case

For customer usage, we should support a way to define gitlab-sites.json that does not require creating the file if using something known.

Scope of Work

• The gitlab-cluster.json should be re-named gitlab-sites.json
• GITLAB_CLUSTER_DESCRIPTION_JSON configuration in gitlab-cluster-rc.sh should be allowed to interpret the following or should be deprecated to allow the following
  • pull gitlab-sites.json from an arbitrary URL
  • pull gitlab-sites.json from a defined reference architecture by a known and documented alias
  • pull gitlab-sites.json from a local file in SECRETS

Contextual Information

Came up during discussions in https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/514

Acceptance Criteria

Users have the above stated options available to configure their site(s).