coreos-kernel: enable support for using ipset with iptables

We enabled ipset support a while back but missed NETFILTER_XT_SET which
is needed for using ipsets in iptables rules. Enable a few other
iptables options we were missing just in case someone wants them.
Merge pull request #869 from marineam/ipset

coreos-kernel: enable support for using ipset with iptables