remove return code that is always zero

No change in logic. Just remove redundant function return code.
remove vestigial function

The function slurm_api_set_conf_file() is never referenced.
Remove it. No change in logic.
retry slurm.conf file

Add logic to sleep and retry if slurm.conf can't be read.
Without this, the slurmd daemons may die and when the SlurmdTimeout
is reached, the nodes will be marked DOWN and their jobs will be
killed.
In the long term, it would be good to exit only if the read files
on program startup, and the daemons keep running with old configuration
on reconfiguration, but I don't have time to do that work now.