Create the images data_bag directory in local mode

This is required to write the image data_bag to disk on the `save` and `ready_image` methods when running `machine_image` against a chef zero instance.

I'm not entirely sure if the provider was actually the best place to do this but it made sense in my head since we had access to `run_context` and could do it before anything else - let me know if we can do it in a better way.

Should solve #228.