cgroups: Add ability to reuse existing cgroup yard directory

Currently we always create temporary directory where we restore
cgroups, but this won't work in case if mounting cgroups is forbidden
from inside of a container for some reason (as in OpenVZ kernel).

So one can pass --cgroup-yard option to specify an existing
directory where cgroups are living. By default we assume it
lays in /sys/fs/cgroup.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>