cgmanager: detect whether cgmanager supports name= subsystems

On older cgmanager the support was broken.  So rather than
fail container starts altogether, just keep the old lxc behavior
in this case by not using name= subsystems.

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