cgmanager: slow down there (don't always grab abs cgroup path)

cgmanager: slow down there (don't always grab abs cgroup path)

When I converted attach and enter to using move_pid_abs, these needed
to use the new get_pid_cgroup_abs method to get an absolute path.  But
for some inexplicable reason I also converted the functions which get
and set cgroup properties to use the absolute paths.  These are simply
not compatible with the cgmanager set_value and get_value methods.
This breaks for instance lxc-test-cgpath.

So undo that.  With this patch lxc-test-cgpath, lxc-test-autotest,
and lxc-test-concurrent once again pass in a nested container.

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