cg: rpc -- Add bindings for custom cgroup props engine

For handling  --cgroup-props, --cgroup-props-file and
--cgroup-dump-controller from RPC interface.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>