In case you want to supply answwers when installing package, here's how you can find wich one are needed by the package:
apt-get install iptables-persistent
apt-get install debconf-utils
debconf-get-selections | grep ^iptables-persistent
echo "iptables-persistent iptables-persistent/autosave_v6 boolean true" | debconf-set-selections
apt-get install iptables-persistent