To access neutron objects we're moving to a dynamic plugin approach as
shown in the depends on and related patches.
This patch removes the imports of neutron.objects and uses neutron-lib
to load them instead. The remaining base object import will be addressed
later.
Depends-On: https://review.openstack.org/#/c/570060/
Change-Id: If1b140d95c039833ed4c144fb5f22328345dcd19