Gres/gpu plugin - If no GPUs requested, set CUDA_VISIBLE_DEVICES=NoDevFiles

This bug was introduced in 2.5.2 for the case where a GPU count was
configured, but without device files. Unfortunately it left the
CUDA_VISIBLE_DEVICES environment variable unset if GPUs were configured,
but the job did not request any of them.
Change to reservation test for some systems
Enable task affinity testing with task/cgroup

Without this change, the testing only happens with task/affinity
Remove vestigial logging messages
Merge branch 'slurm-2.5'