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)
I would probably move this delete out of the Exist function and have the caller determine if they want to delete the rule or not.  I think this is two separate issues, the -C on the check is not always available and then the deletion of duplication rules