Allow alternate binaries for iptables input plugin. (#4682)

etc/telegraf.conf

@@ -2004,6 +2004,8 @@
 #   ## Setting 'use_lock' to true runs iptables with the "-w" option.
 #   ## Adjust your sudo settings appropriately if using this option ("iptables -wnvl")
 #   use_lock = false
+#   ## Define an alternate executable, such as "ip6tables". Default is "iptables".
+#   # binary = "ip6tables"
 #   ## defines the table to monitor:
 #   table = "filter"
 #   ## defines the chains to monitor.