Avoid TestClient error on missing '__mro__' attr

The TestClient unit test fails with:

   AttributeError: class Zeros has no attribute '__mro__'

on Python 2.6 running on Ubuntu Lucid.

Resolve by defining Zeros as a new-style class.

Change-Id: I97f8eaac31d34a68613e9b30a177191a9de281b0

glance/tests/unit/test_clients.py

@@ -1837,7 +1837,7 @@ class TestClient(base.IsolatedUnitTest):
                    'properties': {'distro': 'Ubuntu 10.04 LTS'},
                   }
 
-        class Zeros:
+        class Zeros(object):
             def __init__(self, chunks):
                 self.chunks = chunks
                 self.zeros = open('/dev/zero', 'rb')