Do not pass :default:default iscsi nic specification to dracut (#500273)
Instead leave the iscsi_iface_name and netdev_name values out,
i.e. the option should be:
netroot=iscsi:[<username>:<password>[:<reverse>:<password>]@][<servername>]:[<protocol>]:[<port>][:[<iscsi_iface_name>]:[<netdev_name>]]:[<LUN>]:<targetname>