Rehome IPV6_MODES constants
Rehome IPV6_MODES constants

In preparation for rehoming neutron.api.v2.attributes into
neutron-lib, we need to bring over a few additional
constants that are used in attributes.py.

Change-Id: Ie4184b87ade321794f3979b02cb49a88e2090afa

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib