Change ADES-PBS to use the testing_free PBS server on Pleiades

[jjacob7734]

Change ADES-PBS to use the testing_free@pbspl4 PBS server on Pleiades. That PBS server has lower latency between the queued and running states than the standard queues.

Example from @foobob:

qsub -l select=1:model=bro:ncpus=8:mem=30GB,walltime=1:00:00 -q testing_free@pbspl4

Definition of Done:

• [x] ADES-PBS execute endpoint causes jobs to be submitted via the qsub command and using the testing_free@pbspl4 server
• [x] ADES-PBS status endpoint works as expected via qstat command
• [x] ADES-PBS dismiss endpoint works as expected via qdel command
• [x] Documentation provided
• [x] Github repo commits provided
• [x] Live demo provided

[jjacob7734]

I solved this by making the PBS queue name a setting that can be configured with an environment variable, or overridden by a command line argument in the ADES_PBS constructor. The environment variable setting looks like this: export ADES_PBS_QUEUE=testing_free@pbspl4.