Convert to slapos SMTP settings for gitlab:

    - convert to jinja2
    - remove support for gitlab CI (we do not support it (yet ?))
    - add handling of `smtp_enable` parameter directly to that file
      ( omnibus handles this parameter externally and just removes
        smtp_settings.rb if it is true )

NOTE smtp_settings.rb contains SMTP password, so it is mode is set to 0600.

/cc @kazuhiko, @jerome