Fix gitlab-shell config

The gitlab-shell config was erroneous because the `gitlab_url` configuration was missing the trailing `/gitlab/` path component in the URL.

You can verify this by running `sudo /home/git/gitlab-shell/bin/check`, which in this case resulted in a 500 error:
```
vagrant@gitlab:/home/git/gitlab-shell$ sudo /home/git/gitlab-shell/bin/check
Check GitLab API access: FAILED. code: 500

Check directories and files:
        /home/git/repositories: OK
        /home/git/.ssh/authorized_keys: OK
```

This PR fixes the `gitlab_url` and further improves the template by using node variables.