Apply six for metaclass
Apply six for metaclass

__metaclass__ cannot be used in python3.
six be used in general for python 3 compatibility.

Change-Id: Ib0d71b9676dc01390679396443575c05975d3b58
Closes-Bug: #1236648

citrix-openstack-build/neutron-fwaas,dougwig/x-neutron-fwaas,gaolichuang/neutron-fwaas,openstack/neutron-fwaas,openstack/neutron-fwaas,gaolichuang/neutron-fwaas