[Devel] Improve gitlab-ci
@zmathe here is what you asked for :-)
Two changes:
- We check the version defined in releases.cfg, init.py and Dockerfile when building the release. This allowed me btw to spot that v8r9-pre2 is not correct in the releases.cfg
- When building the tarbal, we take the releases.cfg from the devel branch if the tag contains "pre" otherwise from master
CAUTION: this change will break the possibility to tag version v0r* !!! We should first migrate to the dev cvmfs approach. @fstagni can you comment here when this is done in case I forget ?