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

Tehsmash/networking-cisco,noironetworks/networking-cisco,openstack/networking-cisco,CiscoSystems/networking-cisco,cisco-openstack/networking-cisco,noironetworks/networking-cisco,Gitweijie/first_project,cisco-openstack/networking-cisco,Gitweijie/first_project,nikolay-fedotov/networking-cisco,CiscoSystems/networking-cisco,hareeshpc/networking-cisco,openstack/networking-cisco,Tehsmash/networking-cisco,hareeshpc/networking-cisco