Make lxc-execute without rootfs work.

Make lxc-execute without rootfs work.

That means, don't try to pin a null rootfs, and don't try to mount /proc
since /var/lib/lxc/root/proc doesn't exist to be mounted onto.
The apparmor patches are not yet upstream, so this patch will not go
upstream by itself.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>