Apply six for metaclass
Apply six for metaclass

The way to using metaclass has changed in Python3.

Python 2.7 way:

class Foo(object):
  __metaclass__ = FooMeta

Python 3 way:

class Foo(object, metaclass=FooMeta):
  ...

The six.add_metaclass() decorator allows us to use one syntax that
works for both Python 2.7 and Python 3.

Change-Id: Ibd31a9f91bac6ec220d802f16c77ee0306e211b3
Closes-Bug: #1236648

Shihta/python-novaclient,takeshineshiro/python-novaclient,guptaankita/python-novaclient,guptaankita/python-novaclient,akash1808/python-novaclient,JioCloud/python-novaclient,mat128/python-novaclient,metacloud/python-novaclient,metacloud/python-novaclient,akash1808/python-novaclient,takeshineshiro/python-novaclient,Shihta/python-novaclient,JioCloud/python-novaclient,mat128/python-novaclient,CCI-MOC/python-novaclient,CCI-MOC/python-novaclient