validate maxdelay for cron job

The parameter descriptions says the value should be a positive integer
(or 0). Validate the given value to reject invalid values such as
strings or even negative values.

Change-Id: Ia6e238f85b7eef5b8f7b65ca37481d77a48c378a

manifests/storage/drive_audit.pp

@@ -104,7 +104,7 @@ class swift::storage::drive_audit(
   $monthday                                 = '*',
   $month                                    = '*',
   $weekday                                  = '*',
-  $maxdelay                                 = 0,
+  Integer[0] $maxdelay                      = 0,
   # drive-audit.conf options
   $log_facility                             = 'LOG_LOCAL2',
   $log_level                                = 'INFO',