Nginx: Do not touch nginx_wild_ssl.conf during this upgrade.

This will result with 'Welcome to nginx!' message on all Aegir URLs using by default global HTTPS proxy if the system is old enough to still use IP bases 'listen' directive before the upgrade. It is safe to modify only once the system has been upgraded to BOA-2.3.x, so its /var/aegir/config/server_master/nginx.conf has been automatically updated to use wildcard in the global catch-all server/location.