python: Resolve pep8 comparison errors.

python: Resolve pep8 comparison errors.

Resolve pep8 errors:

  E711 comparison to None should be 'if cond is None:'

The reason comparing against None with "is None" is preferred over
"== None" is because a class can define its own equality operator and
produce bizarre unexpected behavior.  Using "is None" has a very
explicit meaning that can not be overridden.

  E721 do not compare types, use 'isinstance()'

This one is actually a mistake by the tool in most cases.
'from ovs.db import types' looks just like types from the Python stdlib.
In those cases, use the full ovs.db.types name.  Fix one case where it
actually was types from the stdlib.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>