9262: Stop setting sbatch --priority in crunch-dispatch-slurm.

9262: Stop setting sbatch --priority in crunch-dispatch-slurm.

There are a few reasons we want to back this out:

* `--priority` is a relatively new feature of SLURM, added in 14.03,
  and it's not clear how many users are running that.
* Setting `--priority` requires extra privileges, per the sbatch man
  page, and it's not clear crunch-dispatch-slurm should run with
  those.
* There are multiple ways to set the "priority" of a SLURM job.
  There's also a niceness value.  Which is correct to use is probably
  ultimately going to depend on site rules or conventions.  We need to
  get more feedback on what people are doing before we implement
  something.  We might need configurations options along the lines of
  "translate container priority into this option with this scaling
  factor," or something like that.

Per discussion with Tom Clegg.  Closes #9626.