Add product factory

This MR provides support for the product factory directly in logan, removing the need for the wrapping shell script

It adds 3 new config options and version 1.3 of the config

use_product_factory: true
product_vars_path: ./product-vars
product_factory_prefix: gcp-product-factory

It also adds a little code specifically for adding --var-file= when using terraform. Any ideas on making this a little more modular would be welcome as this is the first non docker bit of code in logan

Resolves #21 (closed)