iptables-port-proxy: use -n on iptables -L

Since there's no evident need to have iptables perform reverse DNS
resolution on the entries in the tables, don't do that. It saves time
even when nothing is wrong with DNS config.

Bug 1156200 - oo-admin-ctl-iptables-port-proxy is needlessly slow under DNS failures
https://bugzilla.redhat.com/show_bug.cgi?id=1156200