Log artifact storage and association for pipeline stages

Each stage of assembly creation--unit-test, build-app, deploy-app, and so on--will create log output and possibly other artifacts.
Conveying these artifacts to the user--the logs in case of failure, and the completed DU or its location after a successful run--is very important for their development efforts.

At present neither the worker nor the proposed unittest stage store the output of their executions anywhere for users to consume them. I propose a method of storing the logs for later retrieval or consumption by a suitable management tool.

(Forthcoming; I will submit the spec to our spec repo)