Restore default get_njobs_in_queue behavior

[esoteric-ephemera]

The get_njobs_in_queue function of CommonAdapter was recently updated in v2.0.4 to use shell=True all the time when calling qstat. That seems to not work in practice, as running:

python -c 'from fireworks.user_objects.queue_adapters.common_adapter import CommonAdapter ; print(CommonAdapter("SLURM").get_njobs_in_queue())'

returns say thousands of jobs, which is the total number of jobs running in the queue, rather than the number I expect to see. (Specifying the username explicitly also doesn't work.)

Just making this optional, with the default behavior set to shell=False