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/xtrabackup/server/single.yml b/xtrabackup/server/single.yml
index f72a92a..ad06a94 100644
--- a/xtrabackup/server/single.yml
+++ b/xtrabackup/server/single.yml
@@ -4,3 +4,9 @@
   _param:
     xtrabackup_qpress_source: pkg
     xtrabackup_qpress_source_name: qpress
+  linux:
+    system:
+      cron:
+        user:
+          xtrabackup:
+            enabled: true