Change AWS credentials with role checking for S3 integration

[smaniotto]

The Terraform config introduced on #44 uses an authorized role with write access for the app S3 bucket. This way we don't need to use AWS credentials as env vars.

If we use Heroku as infra provider we still need these credentials.

We don't have this on the boilerplate yet, but since 99% of the apps use it, it makes sense adding it in my opinion. The suggestion would be to keep some sort of:

if env("AWS_ACCESS_KEY_ID"):
    AWS_ACCESS_KEY_ID = env("AWS_ACCESS_KEY_ID")
    AWS_SECRET_ACCESS_KEY = env("AWS_SECRET_ACCESS_KEY")

Ideas?