Enable IPv6 support in ferm firewall

'ferm' role can now manage 'ip6tables' alongside 'iptables' rules. IPv6
support is currently disabled by default, but can be enabled by adding
'ip6' to list of enabled ferm filter domains. After that, ferm will
manage both tables with correct IP addresses / networks in each.

Make sure that all roles which manage their own ferm rules are updated
to be IPv6-aware before enabling IPv6 support.