Define a registry adapter to use when discovery is not possible, i.e. just list the images.

[jwmatthews]

For those cases where a user wants to use the Broker against a Registry that does not support discovery, let's provide a clear option of explicitly listing the images.

Assume we will want to call this adapter something that clearly expresses it is a manual listing of images, i.e. no discovery. User is responsible for listing the desired images to obtain.

Will need documentation to explain that if we lack working adapters for their intended registry choice, then it's possible to fallback to this method manually listing images.

I'm assuming.....that if we have the image name we can speak to the registry to get the manifest and obtain the ServiceBundle spec....unsure if that is always true.