Updated openstack/openstack
Project: openstack/heat  cc69c4fca7e64bfda7047f53d3b4c26c732bb055

Use six.add_metaclass instead of __metaclass__

According to http://docs.openstack.org/developer/hacking/ it should
be preferred to use six.add_metaclass instead of __metaclass__.

Change-Id: I1775035ea998b78a52c28ffe71dda2ede1377b31