Deployment repository garbage collection

The deployment repository is only loosely coupled to the domain model; i.e. users can add content to the repo and then have a problem or decide to not execute a deployment plan with the result that the deployment is not used in the model. Or they can remove the deployment from the model via an update, which doesn't remove it from the repo (in case the update needs to be rolled back.)

Effect of this is unreferenced deployment content can accumulate in the repo, so we need a gc utility that can be used to clean it out.