Convert gitlab-shell configuration file to slapos:

    - convert to jinja2,
    - connect gitlab-shell to unicorn & redis unix sockets

NOTE

    - http_settings are left to be default (empty) ones - as that works ok.
    - `auth_file` is still configured to point to wont-be-used sshkeys
      file, as without it gitlab-shell check will fail.
    - support for audit_usernames and git_annex is disabled and
      remains not configurable.

/cc @kazuhiko, @jerome