python: Resolve a deprecation warning.

python: Resolve a deprecation warning.

Resolve the following deprecation warning.  This deprecation warning
advises to use an alternative syntax which is also compatible with
Python 3, where has_key() was removed.

  W601 .has_key() is deprecated, use 'in'

Also fix this related error:

  E713 test for membership should be 'not in'

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