Discard designs/nfv

This branch removes `designs/nfv/nfv` in favor of `designs/neutron/snabbnfv-bench`. The `loadgen` benchmark now uses `snabbnfv-bench`, which is a benchmarking wrapper around `lib.nfv.config` (e.g. uses the same code paths as, `snabbnfv-traffic` does).

Consider this a work in progress, as I haven't been able to get more than 1.7 Mpps out of it yet. Thoughts on this appreciated.