Network setup in add-ons

Add-ons can be also added via new boot option addon=<i>URL</i> (see https://fate.suse.com/314318).

Unfortunately this does not work correctly for remote add-ons when installingn from DVD, network is not configured in this case and this results in an error when accessing the server.

The fix calls inst_check_network client which asks the user for network setup if network is not running.