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)
That works and I swapped it out, I was also thinking along the lines of if there was a way to check if the string contains the substring via regex all in one swoop. Because this feels a bit nasty.