python: Convert dict iterators.

python: Convert dict iterators.

In Python 2, dict.items(), dict.keys(), and dict.values() returned a
list.  dict.iteritems(), dict.iterkeys(), and dict.itervalues() returned
an iterator.

As of Python 3, dict.iteritems(), dict.itervalues(), and dict.iterkeys()
are gone.  items(), keys(), and values() now return an iterator.

In the case where we want an iterator, we now use the six.iter*()
helpers.  If we want a list, we explicitly create a list from the
iterator.

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