[CI] Alias for services

If you use services with custom docker registry the host become unreadable (registry.gitlab.com__gitlab-org__test-elastic-image) As docker supports aliases natively --link real_host:alias_host it would be great to have an ability to specify the alias just like following example

- "registry.gitlab.com/gitlab-org/test-elastic-image->elasticsearch"

or maybe in some other way.

/cc @tmaczukin @ayufan