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: Ib9f90c88f564ac5a0d545f955257741290998a5e

manifests/expirer.pp

@@ -46,7 +46,7 @@ class aodh::expirer (
   $monthday                          = '*',
   $month                             = '*',
   $weekday                           = '*',
-  $maxdelay                          = 0,
+  Integer[0] $maxdelay               = 0,
   $alarm_histories_delete_batch_size = $facts['os_service_default'],
 ) {