Enabling service's users in /etc/cron.allow

When https://gerrit.mcp.mirantis.net/25051 is merged only
users added to `linux:system:cron` pillar will be allowed
to use crontab tasks.

This commit adds the following users:
* aodh
* cassandra
* ceph
* elasticsearch
* glance
* keystone
* panko
* xtrabackup
* zookeeper

Enable managing cron packages/services and add 'root' user
to both `/etc/at.allow` and `/etc/cron.allow`.

Related-Prod: PROD-22546

Change-Id: Id543d0906842955c5632b1efa6985186afa6c9a9
diff --git a/panko/server/cluster.yml b/panko/server/cluster.yml
index d641f36..3a4cb65 100644
--- a/panko/server/cluster.yml
+++ b/panko/server/cluster.yml
@@ -11,6 +11,12 @@
     panko_memcached_node03_address: ${_param:cluster_node03_address}
     # Keep events in database for 30 days
     panko_event_time_to_live: 2592000
+  linux:
+    system:
+      cron:
+        user:
+          panko:
+            enabled: true
   panko:
     server:
       role: ${_param:openstack_node_role}
@@ -32,4 +38,4 @@
       expirer:
         cron:
           minute: 0
-          hour: 2
\ No newline at end of file
+          hour: 2