Add support for GPU in Incus

[etiennedub]

Add support for GPUs in Incus by explicitly passing the PCI IDs of the GPU devices as a list.

This has multiple limitations:

1. The NVIDIA driver and the NVIDIA Container Toolkit must be installed on the Incus host.
2. It only works with unprivileged containers (there is a validation in the code for this).
3. The node count must be 1 (no validation is currently enforced).

[etiennedub]

I initially modified the Puppet logic for the driver, but realized we can simply omit the gpu tag instead. The gpu tag is only used for the NVIDIA driver, and the Slurm logic is independent of it.