The previous introduction of different pipelines wip/mr/develop/release at .gitlab-ci.yml level did not worked exactly as expected. For instance, a single commit on a merge request has trigged 2 pipelines: 436 as "wip"and 437 as "mr".
I propose to make this detection in the main_ci.cmake, which will simplify the code in yml.
The use of ccache is removed as it doesn't work out-of-the-box. The runners have to configure an access to S3 or GCS in order to store their cache between jobs. At the moment, the cache is kept locally and deleted when the container ends. An other way to store the ccache content may be investigated in the future (the cache setting from GitlabCI is only usefull for permanent runners).
