Make sure both next_server and netmask are defined.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Add header for fai-host.conf file.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>