refactor remaining code to manifest

Now it's fully manifest approach with the naming, thinking and
everything.

Makes a lot of things clearer and should help getting multiarch done.

In theory this is sufficient going forward, since manifests can also be
used for single-arch. In order to be backwards compatible and still
support the simple tagging approach we can re-introduce the tagging
part and split it by using different CI templates, while the
build-ah-engine uses the multiarch templates.

For compatibility reasons I'd use a new template name for multiarch in
that case.