This requires us to check that at cgfsng_ops_init, rather than
cgfs_init.  Cache the hierarchy and cgroup.use info globally
rather than putting it into the per-container info, as cgmanager
does.  This is ok as both cgroup.use and the list of usable
hierarchies are in fact global to a lxc run.

Closes #952
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>