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/linux/system/single/simple.yml b/linux/system/single/simple.yml
index 2b4e919..45be219 100644
--- a/linux/system/single/simple.yml
+++ b/linux/system/single/simple.yml
@@ -8,6 +8,16 @@
     local_package_repos: false
   linux:
     system:
+      at:
+        enabled: true
+        user:
+          root:
+            enabled: true
+      cron:
+        enabled: true
+        user:
+          root:
+            enabled: true
       local_package_repos: ${_param:local_package_repos}
       timezone: UTC
       locale: