Introduce localnet / physnet topology

Add another topology type to the `NetConf.Topology` configuration. It should be named localnet / physnet - or come up with a better name.

Whenever a new network attachment definition is added, if it points to another network, we must update the ovn bridge bindings accordingly, explaining through which OVS bridge will the traffic flow.

Finally, a localnet port must be added to the flat L2 network, whenever the localnet topology is requested.

Definition of done:

• PRs implementing the above merged
• e2e tests asserting these work merged