Emit `-N` in Slurm job script

TACC requires the `-N` option to be emitted in the job script, but ReFrame does not emit it currently. We should give the option to enable that. The workaround currently is the following:

```python
@rfm.run_after('setup')
def set_num_nodes(self):
    self.job.options = [f'-N {self.num_tasks // self.num_tasks_per_node}']
```

FYI @rtmclay