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/glance/control/cluster.yml b/glance/control/cluster.yml
index e218835..ae2b3c1 100644
--- a/glance/control/cluster.yml
+++ b/glance/control/cluster.yml
@@ -6,6 +6,10 @@
 parameters:
   linux:
     system:
+      cron:
+        user:
+          glance:
+            enabled: true
       package:
         python-pymysql:
           fromrepo: ${_param:openstack_version}