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: I33a0d1b0a0a31beacc8c768eb18fac3cb9ed5936
Merge "Use six.add_metaclass instead of __metaclass__"