title: Unable to start filebeat.error missing fileld
body: <p>Below is my filebeat.yml file , I am unable to start service getting error</p>
  
  <pre><code>filebeat.prospectors:
  - input_type: log
  ** paths:**
  ** - /opt/apache-tomcat-7.0.82/logs/*.log**
  
  document_type: apache-access
  fields_under_root: true
  
  output.logstash:
  ** hosts: '${host}'**
  </code></pre>
  
  <p><strong>host is environment variable i did export host="10.2.3.1:5044"</strong></p>
  
  <pre><code>Apr 10 06:59:35 node1 filebeat[401]: Exiting: error initializing publisher: missing field accessing 'output.logstash.hosts' (source:'/etc/filebeat/filebeat.yml')
  Apr 10 06:59:35 node1 systemd[1]: filebeat.service: main process exited, code=exited, status=1/FAILURE
  Apr 10 06:59:35 node1 systemd[1]: Unit filebeat.service entered failed state.
  Apr 10 06:59:35 node1 systemd[1]: filebeat.service failed.
  Apr 10 06:59:35 node1 systemd[1]: filebeat.service holdoff time over, scheduling restart.
  Apr 10 06:59:35 node1 systemd[1]: start request repeated too quickly for filebeat.service
  Apr 10 06:59:35 node1 systemd[1]: Failed to start filebeat.
  Apr 10 06:59:35 node1 systemd[1]: Unit filebeat.service entered failed state.
  Apr 10 06:59:35 node1 systemd[1]: filebeat.service failed.``
  </code></pre>