Direct fog url from aws configuration

Use aws configurations to get fog url instead of `public_url`, to bypass Carrierwave CDN configuration and talk
directly to fog provider (in the given scenario, to upload directly to provider).

We can remove the `public_url_by_aws_configuration` method if this Carrierwave [PR](https://github.com/carrierwaveuploader/carrierwave/pull/1225) creating a public api is accepted