title: Run single instance of oozie coordinator workflow
body: <p>I have a oozie coordinator which runs every 30 minutes.</p>
  
  <p>But I want to run only one instance of it at a time.</p>
  
  <p>(Say, if the workflow takes more than 30 minutes to complete, I don't want the next scheduled workflow to start running until the previous one finishes)</p>
  
  <p>Is there a configuration property I can set in coordinator.xml to achieve this?</p>