This is a subset of the changes for implementing the floating IP
port forwarding feature in neutron, using OVN as the backend.
This changeset introduces functional tests for the IDL functions
added to the NB API for supporting port forwarding.
Depends-On: https://review.opendev.org/#/c/741000/
Change-Id: I01f54ea4a21eb635f0bf87a15694929ed1f3a643
Partially-implements: ovn/port_forwarding
Partial-Bug: #1877447