Validate status of cron resource

This makes sure that the supported value is used for the parameters
which determine the ensure property of a cron resource type.

Change-Id: I1abe90a8f060804c6cc1660158360dfeac4495ad

manifests/cron/fernet_rotate.pp

@@ -54,7 +54,7 @@
 #   Defaults to $::keystone::params::user
 #
 class keystone::cron::fernet_rotate (
-  $ensure   = present,
+  Enum['present', 'absent'] $ensure = present,
   $minute                           = 1,
   $hour                             = 0,
   $monthday                         = '*',

manifests/cron/trust_flush.pp

@@ -61,7 +61,7 @@
 #   Defaults to $::keystone::params::user
 #
 class keystone::cron::trust_flush (
-  $ensure           = present,
+  Enum['present', 'absent'] $ensure = present,
   $minute                           = 1,
   $hour                             = '*',
   $monthday                         = '*',