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: I08703a8b2927f45f4705fe085368d1ebd78b02fa
Closes-Bug: #1236648

keystoneclient/base.py

@@ -216,11 +216,10 @@ class Manager(object):
             return self.resource_class(self, body[response_key])
 
 
+@six.add_metaclass(abc.ABCMeta)
 class ManagerWithFind(Manager):
     """Manager with additional `find()`/`findall()` methods."""
 
-    __metaclass__ = abc.ABCMeta
-
     @abc.abstractmethod
     def list(self):
         pass