doc: Add description about ignoring lxc.cgroup.use when using cgmanager

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
Fix the unprivileged tests cgroup management

To cover all the cases we have around, we need to:
 - Attempt to use cgm if present (preferred)
 - Attempt to use cgmanager directly over dbus otherwise
 - Fallback to cgroupfs

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