Metaclass Python 3.x Compatibility

According to the OpenStack style guide
(http://docs.openstack.org/developer/hacking/),
for Python 3.x compatibility use "six.add_metaclass"
instead of " __metaclass__."

Closes-Bug: #1236648
Change-Id: I2f507a90ee08cc05bd764549002a4656c187b6fc
Merge "Metaclass Python 3.x Compatibility"