More graceful job cancellation

Hey all,

This is just a thought for the `SLURMCluster` for now (since that's what I'm familiar with) but similar options may be available in other clusters too. Currently, the `cancel_command` in the `SLURMJob` class is a bare `"scancel"`.

https://github.com/dask/dask-jobqueue/blob/8713202488c664452bf0883bcd4f776536644676/dask_jobqueue/slurm.py#L15

This means that, even when workers are shutdown completely gracefully, the Slurm job is marked as `CANCELLED`. Instead, if the command were `scancel --signal=SIGTERM` the job would be marked as `COMPLETED`. Its possible there could be cases where we would want a job to cancelled, which complicates this somewhat.

In the simple case, however, I think this could be implmented with a simple change of `cancel_command` to:
```python
class SLURMJob(Job):
# Override class variables
submit_command = "sbatch"
cancel_command = "scancel --signal=SIGTERM"
config_name = "slurm"
```

It'd be great to get some more thoughts on the implications for this.