MAR-189: Rework fix for legacy iptables and ferm:

- Unfortunately, using diversions with iptables legacy binaries does
not work correctly because the iptables package will try to run
update-alternatives on install/upgrade, and error out because the
files are not available in the original locations.
- Divert the ferm binary instead, and roll-out a custom patched
version of it instead.
- Use a custom script to drop the legacy iptables (for both IPv4 and
IPv6).
- Update the tests accordingly.