cgmanager: chown cgroups to the container root

After this patch, starting an unprivileged container using
cgmanager gets the cgroup chown to the container root, so
that it can install the cgmanager (proxy) and make cgroup
requests.

(Still desirable and not in this patch is the automatic setup of
/sys/fs/cgroup/manager/sock, which you can currently do with
two lxc.mount.entries)

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