Provide custom generic Slurm flags from CI environment

[byjtew]

Instead of defining each Slurm variable manually in the driver/executor, we should provide a generic way to declare custom Slurm parameter(s) using the CI environment variables.

For instance: CI_SLURM_GENERIC__GRES="gpu:4"
Adding automatically the line: #SBATCH --gres=gpu:4 in the generated sbatch file.