There need to be some tool which will be able to revert migration from ML2/OVS to ML2/OVN if e.g. something would go wrong during the migration.

As first step, lets focus only on the simple case which is revert of the migration if there were e.g. no new instances spawned on the ML2/OVN yet, so all is plugged still to the br-int as it was before migration.
In such case migration should:
1. Revert network type and mtu changes done in the neutron db,
2. prepare tripleo images with ML2/OVS,
3. remove ovn "agents" from the nodes,
4. We will see what more could be maybe needed...


This procedure will be very basic as first step. Later we can move on with some more complex use cases if that will be needed.