local-parallel queue is not setup well

The setup of the current local-parallel configuration does not work as expected, for several reasons: https://gitlab.idiap.ch/bob/bob.pipelines/-/blob/d8162ffc4fa072a14a8a4d7ac3b558de464a56ef/bob/pipelines/config/distributed/local_parallel.py#L10

1. When we set processes=False, we will only use the python threading module, which will effectively limit the CPU usage to around 100% (i.e., one core), no matter how many cores we use. Only with processes=True, we will get real parallelization.

2. Selecting all possible CPUs via cpu_count() by default does not work well. I have a machine with 128 CPU cores, so setting up all 128 cores takes longer than an experiment -- especially when using processes=False above, I commonly get a timeout error.

Before, we had something like local-p4 with 4 parallel cores, and alike. I think it would be a good idea to incorporate several of these here. Are there any objections?