cgmanager: have root escape to root cgroup before starting

If a user in cgroup /a/b/c does 'lxc-start -n u1', then u1
should be started under /a/b/c/u1.  However if he does
'sudo lxc-start -n u1', then that cgroup shoudl start under
/lxc/u1.

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