More general for all list options.

Seems to currently affect:
lxc.network (clear all NICs)
lxc.network.* (clear current NIC)
lxc.cap.drop
lxc.cap.keep
lxc.cgroup
lxc.mount.entry
lxc.mount.auto
lxc.hook
lxc.id_map
lxc.group
lxc.environment
Signed-off-by: Marko Hauptvogel <marko.hauptvogel@googlemail.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>