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: I91d029250da223f999381d594a3c7b953ec19898

manifests/cron/db_purge.pp

@@ -69,7 +69,7 @@ class cinder::cron::db_purge (
   $user                             = $::cinder::params::user,
   $age                              = 30,
   $destination                      = '/var/log/cinder/cinder-rowsflush.log',
-  $maxdelay    = 0,
+  Integer[0] $maxdelay              = 0,
   Enum['present', 'absent'] $ensure = 'present',
 ) inherits cinder::params {