Direct fog url from aws configuration

[nvieirafelipe]

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 creating a public api is accepted

[nddeluca]

@nvieirafelipe Thanks for the contribution. I'll wait a bit to see where the PR request goes before merging in. I'm hesitant to include the publish_url_by_aws_configuration method without unit tests.