* update gitlab to latest version
This should refer to the offical docker compose file of gitlab: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/docker/docker-compose.yml
* added option to select gitlab external_url prefix
This is needed so that rancher will put a correct uri scheme prefix to the ${gitlab_hostname} to the external_url.
* added external_url prefix
honestly i don't know if `${gitlab_omnipus_prefix}${gitlab_hostname}` will work since i cannot test it by myself right now, but imo that should work....
* changed version to latest
since image is going to be gitlab/gitlab-ce:latest, the version here inside the readme should reflect this.
* update to latest
