Support building combinations from chunked-without-hash images only

[csweichel]

Chunks without their hash are easy to identify by name. For integrating use with Gitpod we'll need to use the no-hash mode so that we can build combinations that do not require the dazzle.yaml to be present.

We need to add a combination mode --ref which takes chunks from absolute refs and combines them. Prior to combination we should ensure that all those chunks were built from the same base image.