title: date is not appending to elasticsearch index name while using td-agent
body: <p>i need to store the log to Es index when i use logstash date is getting append ed to index name as logstash.2018-08-06,but when i try to give the custom name as in the flowing conf,its not getting added</p>
  
  <pre><code>&lt;/store&gt;
    &lt;store&gt;
    @type elasticsearch
    host X.X.X.X
    port 9200
    logstash_format false
    index_name updatetest.%Y%m%d --&gt; In index name its not replacing with date
    &lt;/store&gt;
  </code></pre>
  
  <p>Here is the index name ,created by above conf updatetest.%Y%m%d --> its should be like updatetest.20180806</p>
  
  <p>Thanks for help in advance</p>