burst_buffer/cray development

Add streamlined test to see if a job has any burst buffer component
Add descripitive error if job node list not a valid cray "nid#" format
Replace _get_bb_size() with _get_bb_spec() to support GRES allocations
Fix GRES info pack for "scontrol get burst"
Add GRES allocations to common data structure
Track global GRES usage as allocations are made and removed
Log stage out failure
Gracefully handle bbs_pre_run error
Add some of the logic for preempting burst buffer allocations for higher priority jobs