Support for parallel workers with individual configurations

Upstream debci supports parallel workers, but they all share the same configuration.

We need a solution where each worker can have its own debci configuration. This is needed for at least two key reasons:

• Assigning specific GPUs to specific workers, on hosts with more than one GPU
• Support for OSes other than Debian (e.g. for Ubuntu, a different debci_mirror is used)

systemd service templates might be a possible solution.