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/zookeeper/backup/server/single.yml b/zookeeper/backup/server/single.yml
index 6796d8a..a97df06 100644
--- a/zookeeper/backup/server/single.yml
+++ b/zookeeper/backup/server/single.yml
@@ -3,6 +3,12 @@
 parameters:
   _param:
     zookeeper_backup_public_key: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCzLUiHKAjfFXiZ3fsgx35uXF6VivfC5WFafu4QMalxmj6W+s277oCfdWA8Du5f1wtQXM73VQ5nHkXhM2UIfUMarsyhXK+BxKVrcgEBNHdKlDytaecUPyuOxTDdGWhN/DPv5/vL8NYWweEYBbNbLgU0Td7Rvm52TUXKThIDjeF7XDxX4ShXWipBSwU4boOUBtR8KWfga8fsqeBN+eacuAQFR3MrrOfVvAuWW6Bsf047cmd+V6Qv0raoW73Nu4M/ZAdTsaR5k62a0cHsSRoi3hCmNRqw+CZaQi8prQU6t26eWPEtznjp5EkPF+LLh8LxUoCfWqWT+Lxe8QQwT1nx/LCN
+  linux:
+    system:
+      cron:
+        user:
+          zookeeper:
+            enabled: true
   zookeeper:
     backup:
       server: