cgcreate, cgconfigparser: Added new parameter for tasks file permissions.

cgcreate, cgconfigparser: Added new parameter for tasks file permissions.

So far cgcreate and cgconfigparser have -f option, which sets permissions
of all files in a group, incl. tasks file. Since we need to put sticky bit
on the tasks file for systemd, this patch adds -s option for this purpose.

The tests are also updated with this change.

Signed-off-by: Jan Safranek <jsafrane@redhat.com>