archlinux template: added sigpwr handling to systemd (lxc-stop)

archlinux is using systemd and systemd's configuration does not have any
services setup to handle sigpwr hook which is sent by lxc-stop command. By
enabling sigpwr service we make sure that lxc-stop will work.

Signed-off-by: Edvinas Klovas <edvinas@pnd.io>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>