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

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](https://github.com/dotnet/dotnet-buildtools-prereqs-docker/pull/363)). 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}