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/single.yml b/panko/server/single.yml
index 497b21e..4ba2787 100644
--- a/panko/server/single.yml
+++ b/panko/server/single.yml
@@ -5,6 +5,12 @@
   _param:
     # 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}
@@ -13,4 +19,4 @@
       expirer:
         cron:
           minute: 0
-          hour: 2
\ No newline at end of file
+          hour: 2