title: Configure Gitlab CI stages to not run in parallel
body: <p>I have a Gitlab CI pipeline with multiple stages. By default, the same stage between two jobs will be run in parallel. One of the stages I have is an integration test stage that should only have one running instance at a time since it relies on specific resources being setup and torn down.</p>
  
  <p>Is there a simple way to specify a certain stage should only have one concurrent instance?</p>
  
  <p>Gitlab Enterprise Edition v10.0.3</p>