Change default slurm arguments to None

[colligant]

Currently the SlurmCluster object builds a slurm script with some default parameters (time=15:00, per_experiment_nb_gpus=1, etc). This PR changes the behavior so users have to specify which slurm directives they want to use. I think there are two reasons for this change:

1. Slurm managed clusters often have sensible defaults for parameters that are not specified in the slurm script.
2. The default parameters aren't mentioned in the docs (unless I missed something), and their configuration caused my script to fail. Debugging was easy enough but I think explicitly specifying needed parameters in the SlurmCluster object is a better choice than setting defaults in the object.

Thanks for this tool!