service: only attempt to load the 'tun' modules if necessary

'tun' support might be compiled into the kernel, thus modprobe will
always fail.

https://mail.gnome.org/archives/networkmanager-list/2014-July/msg00014.html

Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>