Passbolt install script should be more verbose when no `$NGINX_SITES_AVAILABLE`

[WilliamDEdwards]

I install nginx from the http://nginx.org/packages/mainline/debian repository. Their nginx packages do not ship with the /etc/nginx/sites-available directory. Trying to configure nginx with dpkg-reconfigure -phigh passbolt-ce-server fails, because passbolt-ce-server.postinst does nothing when the $NGINX_SITES_AVAILABLE directory is not present (see function configure_nginx_config_file). This is understandable, but confusing, because the installer says nginx is configured and ready to go, even though nothing was done.

[WilliamDEdwards]

I'm finding more strange behaviour when using nginx from nginx's own repository. Maybe this is more of a suggestion to explicitly mention nginx should be installed from the default repository.