Fix duplicate iptables rules

If iptables version is < 1.4.11, try to delete the rule vs. checking if it exists. Fixes #6831.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jfrazelle@users.noreply.github.com> (github: jfrazelle)
probably not necessary, but `[1-9][0-9]{0,2}` might be more appropriate for octets