title: logstash http_poller ssl certification issue
body: <p>I am trying to use logstash http_poller to query a server RESTAPI. I download the server pem through explore, and generate jks file with keytool. but we still get error <strong>"PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target"</strong>. Don't know  what wrong. </p>
  
  <p>The config like below:</p>
  
  <pre><code>http_poller {
      urls =&gt; {  
        restapi =&gt; {
           method =&gt; get
          url =&gt; "https://path_to_resources
          headers =&gt; {
            Accept =&gt; "application/json"
          }
          truststore =&gt; "/path/generated.truststore.jks"
          truststore_password =&gt; "xxx"
          ssl_certificate_validation =&gt; false
          auth =&gt; {
            user =&gt; "xxx"
            password =&gt; "xxx"
          }
        }
      }
      request_timeout =&gt; 60
      interval =&gt; 60000
      codec =&gt; "json"
      metadata_target =&gt; "http_poller_metadata"
    }
  }      
  </code></pre>
  
  <p>By the way, what impact if ssl_certificate_validation is set as false?</p>