Add --wait to iptables commands

Add --wait to iptables commands

This change adds the --wait flag to the insert/update/delete iptables
actions to prevent failures from occuring when iptables is running
outside of puppet. Previously if a script or a user is running iptables
commands while puppet is running, there may be failures in the firewall
provider if iptables cannot get a lock. By default, iptables will just
fail with an error so using --wait will make it wait until a lock can be
established. The --wait is only added for iptables >= 1.4.20.