Adds template for Gitaly storage drain
This adds a template for draining Gitaly storages.
- Epic: https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/935
- Design Documentation: https://gitlab.com/gitlab-com/gl-infra/readiness/-/blob/master/library/gitaly-multi-project/README.md
This is a temporary template that will be removed once production is migrated.