g5k: set correct OAR queue automatically
It's currently hard to reserve machines from multiple queues, see https://gitlab.inria.fr/discovery/enoslib/-/blob/main/docs/tutorials/grid5000/tuto_grid5000_docker_nvidia.py?ref_type=heads#L17
We should figure out the correct queue automatically (still allowing the Enoslib user to set it if they know what they are doing)
This is a bit similar to #158, but more complex: it involves creating multiple jobs, one for each queue. We should use the Providers class for this (exactly like multi-sites reservation).