In Rocky we are going to start working on decoupling neutron db. This
will be a long effort, so from a vmware-nsx POV we can hopefully pick
away at it as we go.
This patch changes to use of class references to fully qulified class
names when defining ORM relationships. This allows us to remove a few
of the neutron db related imports in vmware-nsx.
Change-Id: I1df4cb3eec59488a5b35fac9f943bbfdbb44df63