Cperello/nginx fixes

This branch fixes several issues with the new nginx feature:

- Nginx listening on port 80 conflicting with webserver on 3.0 upgrades to 3.1 (http://trac.zentyal.org/ticket/6657)
- Nginx failing to start when Zentyal is not registered with remote.
- Squid having problems to get the list of black/white list of urls due to an nginx limit with the upload size.
- Remote connection not working once registered because there are pending changes to be saved. I'm not sure this last point is fixed with the best solution (https://github.com/Zentyal/zentyal/commit/8fb2ef553207dc1e14bd55e79f87e88601fcc0c7). The problem is that before nginx, you could configure Apache to use a folder for CA certificates, with nginx, you are only able to use a .pem file with all certificates inside, so it should exist to be able to start nginx, so this save is required to be done once the 'offline' certificate download is completed.