gitlab: Add config options

Three configs were added:

• 'nginx['error_log_level'] = "error"'
  • Only log errors from nginx
• 'letsencrypt['enable'] = false'
  • The default seems to be 'nil' which can lead to problems if you don't have a valid cert for gitlab installations and are running without a reverse proxy.
  • Shouldn't have any effect in this case as we are using traefik. Doesn't hurt to have it explicitly set though.
• 'postgresql['shared_buffers'] = "512MB"'
  • The default seems to be '256MB' but GitLab reccommends to set this to a quarter of the memory gitlab has access to.
  • Mainly to experiment if this makes my gitlab more performant and/or more memory hungry.