Correct gres logic to handle difference in core/cpu count

The gres_plugin_job_test was returning a count of cores available
to a job, but the select plugins was treating this as a CPU count.
This change converts the core count into a CPU count as needed in
the select plugin and changes the comments related to the function
gres_plugin_job_test().
Correction to GRES element selection logic
Merge branch 'slurm-2.5' of https://github.com/SchedMD/slurm into slurm-2.5