Use GITLAB_USER_LOGIN instead of NUTZER variable

I would prefer to tie the machine a user can start / delete to their gitlab login name, provided by GITLAB_USER_LOGIN. This way all startups/teardowns could be provided re-using the same pipeline and we safeguard against accidentally killing a machine while someone is still working on it by re-starting the wrong job.

If needed, machines left unattended can still be killed using the direct approach with admin login to the cloud.

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