Simplify generate CI

We could use the rules:changes to decide when running a job. The main issue is that Gitlab trigger a single pipeline when merge a set of commits. For now it is working because for some reason the prepare-job clone the repository with the topic. But this main not always the case if the pipeline is triggered once the topic is cleared or by schedules. In such case only the modified files in the last commit are used to generate the job. By using rules:changes Gitlab has the pushed context and can decide the files that have changed. Also for schedules the rules:changes is always true and so all the tests are run.

Also we should run checks only on merge request against default branch because backport merge request may fail for no benefit.