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.

component/firewalld/buildout.cfg

@@ -125,6 +125,7 @@ command =
   sed -i 's#python2.7 /usr/bin/firewall-config#${:python} ${firewalld:location}/bin/firewall-config#' ${firewalld:etc-dir}/lockdown-whitelist.xml
 
   sed -i 's#DefaultZone=public#DefaultZone=trusted#' ${firewalld:etc-dir}/firewalld.conf
+  sed -i 's#FirewallBackend=nftables#FirewallBackend=iptables#' ${firewalld:etc-dir}/firewalld.conf
 
   sed -i "s#'/etc/firewalld'#'${firewalld:etc-dir}'#"     ${firewalld:python-egg}/firewall/config/__init__.py
   sed -i "s#'/usr/lib/firewalld'#'${firewalld:location}/lib/firewalld'#" ${firewalld:python-egg}/firewall/config/__init__.py