Update katello models to support docker v2
Katello's internal representation of a Docker repo currently is based on the v1 registry concepts of images and tags.
Registry v2 uses tags, manifests, and blobs. This ticket is to track the work to update the models for tags and manifests, and possibly include blobs if it makes sense.
fixes #13479 - docker v2 - add model for manifests and update model for tags
This commit contains some initial changes to support the docker v2 model.
This includes the introduction of a new model (manifests) and modifications
to the existing docker tags model.
With docker v2, the docker image model will no longer be needed; however,
removal of it and any associations will be done separately from this commit.