This is a subset of the changes for implementing the floating IP
port forwarding feature in neutron, using OVN as the backend.
This changeset covers the core implementation for portforwarding/drivers/ovn,
mech_driver, ovn-router as well as a subset of tests.
Port forwarding support in ovn_db_sync is not included here to facilitate review.
That, as well as all other supporting changes, are under the ovn/port_forwarding topic:
https://review.opendev.org/#/q/topic:ovn/port_forwarding+(status:open+OR+status:merged)
Depends-On: https://review.opendev.org/#/c/726478/
Partially-implements: ovn/port_forwarding
Partial-Bug: #1877447
Change-Id: I019fe11ac1ddcf2304f3f144c62d52667fc11dce
d74f409c82