Convert old style class to new style class

This patch converts one old style class to new
style class and relplaces Exception.message with
six.text_type(Exception).

Change-Id: I84d36a92f7fbd55db44ce24b43f593be46be5ea5

designate/backend/impl_nsd4.py

@@ -103,5 +103,5 @@ class NSD4Backend(base.Backend):
             self._execute_nsd4(command)
         except exceptions.Backend as e:
             # If domain is already deleted, don't reraise
-            if "not found" not in str(e.message):
+            if "not found" not in six.text_type(e):
                 raise

designate/tests/test_backend/test_nsd4.py

@@ -29,7 +29,7 @@ from designate.tests import resources
 from designate.backend import impl_nsd4
 
 
-class NSD4ServerStub:
+class NSD4ServerStub(object):
     recved_command = None
     response = 'ok'
     keyfile = os.path.join(resources.path, 'ssl', 'nsd_server.key')