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: Ica59437af00ea05335c934e6c3283e9207085587

manifests/cron/db_purge.pp

@@ -67,7 +67,7 @@ class manila::cron::db_purge (
   $user                             = $::manila::params::user,
   $age                              = 0,
   $destination                      = '/var/log/manila/manila-rowsflush.log',
-  $maxdelay    = 0,
+  Integer[0] $maxdelay              = 0,
   Enum['present', 'absent'] $ensure = 'present',
 ) inherits manila::params {