Support fetching AWS IAM credentials for container registry
Zendesk: https://gitlab.zendesk.com/agent/tickets/103377
Docker Distribution supports pulling S3 IAM credentials (see https://docs.docker.com/registry/storage-drivers/s3/). It looks like we need to allow both the option to leave the key and secret to be blank to pull from the instance's credentials (may already be supported) or to omit the key/secret altogether in order to pull from the IAM role.
A customer tried leaving them blank in gitlab.rb
but it didn't work for them. In this case I assume that omitting them would have worked.