Attribute which allow you to define from what cookbook get template.

Attribute `node["simple_iptables"]["template_cookbook"]` allows you to specify where to get a template for generating iptables rules.
`iptables-rules.erb` template contains four iptables tables: `filter`, `nat`, `mangle` and `raw` in the current implementation. But sometimes i must provision servers without someone of these tables and i want to use my own `iptables-rules.erb` template for this.