Add qos option to slurm adaptor

[daub1]

Allow specifying the qos parameter for slurm via the batch block.

batch:
    type: slurm
    ...
    qos: standby

Leading to the additional header line:

#SBATCH --qos=standby

[FrankD412]

@daub1 -- @youngjeffrey and I talked and he's going to take a look at this. It should be rather straightforward to implement two options: 1. a default in the options and 2. a per-step QOS setting.

Usually the batch section is reserved for setting defaults, so is useful if you want to run a whole workflow with a specific quality of service, but lacks the flexibility to do per-step configuration. We're planning to add both so that you can choose what you'd like to configure it to be.