Slurm scheduler broken

The slurm scheduler no longer seems to work. This is likely due to the changes to the interaction with the config.ini file recently. The error is

File "/home/zyou/.conda/envs/bilby/lib/python3.7/site-packages/bilby_pipe/slurm.py", line 31, in write_master_slurm
    ["--{}".format(arg) for arg in self.scheduler_args.split()]
AttributeError: 'NoneType' object has no attribute 'split'

I'm not sure what the cause is at the moment.

We need to

• Fix the bug
• Add a test which checks that a slurm job at least builds (even if it doesn't execute)