add option to conditionally not manage wp-config.php at all

We're spinning up WP in AWS, using pre-baked images and autoscaling. As a result, there's a lot of custom stuff in our WP config, *and* the key/salt values should be the same across all instances. This adds an option to simply not manage wp-config.php at all, and allow a separate (custom, local) module to drop it in place from a template or static file.