Check for disabled OpenVPN instances in openvpn_resync_if_needed

It is much cleaner if we check first for disabled OpenVPN instances, before diving into all the other checks. Note, the final openvpn_resync() does call other routines that finally check if the instance is disabled, but there are so many checks here for various conditions to be met before calling openvpn_resync that it looks better (safer) to bail out early if the instance is disabled.