Add a runit service to monitor repository storages

This process makes a request to unicorn running on the node through the configured socket.

The request will trigger a health check and report back the result in the service log.

The service is an executable in the rails app that lives in bin/

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15426 GDK MR: gitlab-development-kit!404 (merged)