fixed error of NetworkSegment
fixed error of NetworkSegment

In latest neutron code, following error is occurred;
E: 78,39: Module 'neutron.plugins.ml2.models' has no 'NetworkSegment' member (no-member)

Change-Id: If15b798948c407e23ce31896455c42d3d4cb30c0

stackforge/networking-nec,stackforge/networking-nec,openstack/networking-nec,openstack/networking-nec