cgroup: rpc -- Fix @manage_cgroups option parsing

In commit c7d646af we introduced cgroup resotre
modes but when option passed via RPC code it simply
either true or false which erroniously maps to
CG_MODE_PROPS or CG_MODE_IGNORE modes.

Lets map @true to CG_MODE_SOFT to preserve backward
compatibility and enhance this option in future via
separate option.

Reported-by: Ross Boucher <rboucher@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>