title: Can you use environment variables in config file for fluentd
body: <p>I was wondering how to use env vars in the Fluentd config, I tried:</p>
  
  <pre><code>&lt;match **&gt;
  type elasticsearch
  logstash_format true
  logstash_prefix $ENV_VAR
  host ***
  port ***
  include_tag_key true
  tag_key _key
  &lt;/match&gt;
  </code></pre>
  
  <p>but it doesn't work, any idea?</p>