Add new parameter type “maybe string”

Before strict checking was implemented, NIC IP addresses could be set
to “None”. Commit bd061c35 added more strict checking, including
enforcing the IP address to be a string. With this new type, it
can again be set to None.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>