The neutron code is already shimmed to use neutron-lib for the
neutron.db._resource_extend module [1].
This patch switches the code over to use neutron-lib for resource_extend
rather than neutron.
[1] https://review.openstack.org/#/c/624179/
Change-Id: I594dab9e0d207d2565eb70093163b9d8a90602e1