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.