Computational cluster: in non-billable system when 2 jobs are taken by 1 worker, then the second worker does not down-scale

[sanderegg]

If 2 jobs are started in a non-billable system:
-> 2 workers are created,
1 worker takes all the jobs because it can,
-> the 2nd worker is not removed until the end of the computation.

[sanderegg]

closed by #6898