Added a simple template for use with iptables

This change allows someone to open up a iptables port using the varnish recipe within their recipe:

```ruby
iptables_rule "port_varnish" do
    cookbook  "varnish"
    variables :port => 80
end
```

I was hoping to set it up to use the `VARNISH_LISTEN_PORT` but that was beyond my abilities for the time being.