Follow-up on NETOBSERV-1831: MAC-based indexing can be problematic in some cases; it doesn't work well for instance with OVN-kubernetes as a primary network, because MACs aren't found to be unique and don't always seem reliable (cf screenshots)

We need to allow more configuration on which index to use, depending on the secondary network in use. This will also allow to handle more complex cases when both MAC/IP/interface ID should be used.