Add __ne__ built-in function

In Python 3 __ne__ by default delegates to __eq__ and inverts the
result, but in Python 2 they urge you to define __ne__ when you
define __eq__ for it to work properly [1].There are no implied
relationships among the comparison operators. The truth of x==y
does not imply that x!=y is false. Accordingly, when defining __eq__(),
one should also define __ne__() so that the operators will behave
as expected.
[1]https://docs.python.org/2/reference/datamodel.html#object.__ne__

Change-Id: I4c1b630d14030c9e451e58342b0d44e2058fa51f

mistral/db/sqlalchemy/model_base.py

@@ -60,6 +60,9 @@ class _MistralModelBase(oslo_models.ModelBase, oslo_models.TimestampMixin):
 
         return True
 
+    def __ne__(self, other):
+        return not self.__eq__(other)
+
     def to_dict(self):
         """sqlalchemy based automatic to_dict method."""
         d = {}