Fix class name for ServerGroupAffinityFilter

The class name for ServerGroupAffinityFilter is
ServerGroupAffinityFilter and not ServerGroupAntiAffinityFilter. The
wrong class lead to a wrong display on
http://docs.openstack.org/developer/nova/devref/filter_scheduler.html

Change-Id: I690a60cb70eab4837544ddd0bff6f1264e639592
Closes-Bug: #1360375
Merge "Fix class name for ServerGroupAffinityFilter"