title: icinga event command attribute missing "execute" error with v2.3.9
body: <p>I'm using icinga v2.3.9 on centos7</p>
  
  <p>while configuring an EventCommand object, </p>
  
  <pre><code>[2018-12-14 12:34:03 +0000] information/ConfigItem: Committing config items
  [2018-12-14 12:34:03 +0000] critical/config: Error: Required attribute is missing: Object 'restart_service' (Type: 'EventCommand') at /etc/icinga2/zones.d/global-templates/eventcommands.conf:2 -&gt; Attribute 'execute'
  [2018-12-14 12:34:03 +0000] critical/config: 1 error
  </code></pre>
  
  <p>[root@server icinga2]# cat zones.d/global-templates/eventcommands.conf</p>
  
  <pre><code>object EventCommand "restart_service" {
    command = [ PluginDir + "/restart_service" ]
  
    arguments = {
      "-s" = "$service.state$"
      "-t" = "$service.state_type$"
      "-a" = "$service.check_attempt$"
      "-S" = "$restart_service$"
      "-H" = "$host.name$"
    }
  
    vars.restart_service = "$procs_command$"
  }
  }
  </code></pre>
  
  <p>Am I missing something, this config works in v2.10.2-1</p>