letsencrypt support for gitlab-rails

See https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1096 for more information.

Support is not implemented for the remaining http services (e.g. pages). That will require some light refactoring of the current script, to be able to pass the the domain and paths as arguments. The attributes for the acme challenges directory and the acme client key should be made global across the services. To avoid compounding the problem of duplication of external_url parsing among the services, refactoring that first would be a good idea too.