Fixes bug #910295
The None, True, and False values are singletons.
All variable *comparisons* to singletons should use 'is' or 'is not'.
All variable *evaluations* to boolean should use 'if' or 'if not'.
"== None", "== True", "== False", and "!= None" comparisons in sqlalchemy's
where(), or_(), filter(), and_(), and select() functions should not be changed.
Incorrect comparisons or evaluations in comments were not changed.
Change-Id: I087f0883bf115b5fe714ccfda86a794b9b2a87f7
88ccade9d5