Add support for disabling the builds for an image within the manifest.

[MichaelSimons]

There have been times when users need to exclude images from a build. This typically happens when there is a regression triggered by an external dependency (example). The current workflow is to delete the image from the manifest file. This is rather inconvenient because once the issue gets resolved the deleted content is re-added. It would be convenient if users could flag an image to be excluded from the build (e.g. "disabled": true}