Cray burst buffer development

Add new burst buffer error: ESLURM_INVALID_BURST_BUFFER_REQUEST
Add new burst buffer plugin function bb_p_job_validate(), called
  after the job_id is set and write the code to set up Cray environment
  files for the job.
Add stage-in logic
Add logic to claim burst buffer
Add cray logic to claim buffer when job resources selected
Add logic for stage-out and teardown