title: fluent custom plugin registration throws "Unknown output plugin"
body: <p>I am trying to modify an existing Fluentd plugin and use it with my customization. So I :</p>
  
  <ol>
  <li>git cloned the plugin repo from github. <code>git clone &lt;url&gt;</code></li>
  <li>Edited my changes</li>
  <li><strong><code>gem build plugin.gemspec</code></strong></li>
  <li><strong><code>gem install plugin.gem</code></strong></li>
  <li>Created a config mentioning the type of plugin</li>
  <li>Ran <strong><code>fluentd -c &lt;fluend_conf.conf&gt; -vv</code></strong></li>
  </ol>
  
  <p>And I ended up getting this in fluentd log(<strong>fluent-plugin-swift</strong> is the plugin I edited and the type of the plugin is <strong>'swift'</strong>):</p>
  
  <pre><code>2017-03-22 15:00:27 +0000 [info]: fluent/engine.rb:126:block in configure: 
  gem 'fluent-plugin-swift' version '0.0.1'
  
  2017-03-22 15:00:27 +0000 [error]: fluent/supervisor.rb:369:rescue in
  main_process: config error file="fluend_conf.conf" error="Unknown output plugin 
  'swift'. Run 'gem search -rd fluent-plugin' to find plugins" 
  </code></pre>
  
  <p><strong>EDIT-1: For clarifications</strong>
  I have also checked the naming convention of the plugin that is to be registered.
  The plugin is present in <code>&lt;base_dir&gt;/lib/fluent/plugin/</code>. 
  Also the name of file is <code>out_swift.rb</code>. And finally
   inside the file, the plugin gets registered as:</p>
  
  <pre><code>Fluent::Plugin.register_output('swift', self)
  </code></pre>
  
  <p>Also is there any way, I can check the list of registered plugins, and the name it uses to get registered to fluend?</p>
  
  <p><strong>NOTE:</strong>
    The same had previously worked for me, I had to re-do this as someone had formatted the machine I had my changes.</p>
  
  <p>Please let me what is wrong in my approach?</p>