make mountcgroups hook to lxc.include aware (v2)

[resending the patch from correct branch]

commit 09ad624693cb0ef41cf246e2876b5f356783b34d introduced lxc.include option
to include other config files. This patch uses it in mountcgroups hook otherwise
lxc-start fails with following error message for the nested containers;

root@oOo:/var/lib/lxc/raring# lxc-start -l debug -o l -n raring -d
lxc-start: command get_cgroup failed to receive response

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>