Use constant 'IS_DEFAULT' from neutron-lib
Use constant 'IS_DEFAULT' from neutron-lib

Change-Id: I51cf78cfd681acca5888fa29cb3e24a4207abd7e

huntxu/neutron,mahak/neutron,openstack/neutron,openstack/neutron,noironetworks/neutron,mahak/neutron,noironetworks/neutron,mahak/neutron,huntxu/neutron,openstack/neutron