Introduce better item getter helper in query.py

And also use it for simple network fields. Currently the Python
library returns jsnull, which is formatted as 'None' by the Python
client, which (IMHO) is not nice, since these fields are strings (some
of them) or "other". An undeclared gateway is easier to diagnose when
properly reported by the server as missing, rather than "existing but
null".

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>