to populate the list of subsystems to use.

Cgmanager can be started with some subsystems disabled (i.e.
cgmanager -M cpuset).  If lxc using cgmanager then uses the
/proc/self/cgroup output to determine which controllers to use,
it will fail when trying to do things to cpuset.  Instead, ask
cgmanager which controllers to use.

This still defers (per patch 1/1) to the lxc.cgroup.use values.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>