Explicitely set systemd service permission

The configuration happends to default to 0600, resulting in systemd
producing this warning message:

```
Configuration file /usr/lib/systemd/system/elasticsearch-es-01.service is marked world-inaccessible. This has no effect as configuration data is accessible via APIs without restrictions. Proceeding anyway.
```

Set read permission for group and others to fix this issue.

Author: smortex

manifests/service/systemd.pp

@@ -179,6 +179,7 @@
       ensure => 'file',
       owner  => 'root',
       group  => 'root',
+      mode   => '0644',
       before => Service["elasticsearch-instance-${name}"],
       notify => $notify_service,
     }