add assignRoleToSecurityGroupOnParent method to call assignRoleToSecurityGroup on the parent object, from unrestricted code.
The permission is necessary on the role definition object, not the parent object.
As a bonus, this will no longuer fail if the acquisition parent is not the real parent.



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@9128 20353a03-c40f-0410-a6d1-a30d3c3de9de