Set SP_VERSION in Auto DevOps template only if not already set

Problem to solve

In https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/Auto-DevOps.gitlab-ci.yml we have the SP_VERSION variable set. But when testing new images, we need to use different images. Instead of making the Auto DevOps template explicit and change the definition manually, it would be great to override this value with a project/group variable.

Proposal

In the Auto DevOps template, check if the SP_VERSION variable exists and set it only if it is not.

What does success look like, and how can we measure that?

If I set a project variable named SP_VERSION, the Auto DevOps template will use that variable in jobs.