Modify slurm.conf and slurmdbd.conf debug level parsing

Modify slurmdbd.conf parsing to accept DebugLevel strings (quiet, fatal,
info, etc.) in addition to numeric values. The parsing of slurm.conf was
modified in the same fashion for SlurmctldDebug and SlurmdDebug values.
The output of sview and "scontrol show config" was also modified to report
those values as strings rather than numeric values.