component/firewalld: Use iptables backend
On older kernels (like default one in Debian 9) firewalld is failing with: 'python-nftables' failed: internal:0:0-0: Error: Could not process rule: No such file or directory when nftables backend is used. As firewald component shall be compatibility layer, it shall work on most different operating system, and for today it seems the way to make it really compatible.