Deduplicate the image variable definitions on .pipeline-test.yml regarding salsa-ci.yml

.pipeline-test.yml (part of the internal pipeline) needs to redefine the image variables. However, that is prone to error, since any meaningful related change in salsa-ci.yml requires a manual action in .pipeline-test.yml. It would be more suitable to deduplicate/simplify those definitions.

Probably we like a specific IMAGE_VERSION per image, e.g. APTLY_IMAGE_VERSION defined in a common place, and then set them either in salsa-ci.yml or in .pipeline-test.yml.