title: Invoking a plugin after parallel step has completed for a stage
body: <p>I have a pipeline stage like so</p>
  
  <pre><code>stage('Unit Tests') {
        steps {
          parallel (
            "Jasmine": {
              sh 'gulp karma-tests-ci'
            },
             "Mocha": {
              sh 'gulp mocha-tests'
            }
          )
        }
      }
  </code></pre>
  
  <p>I would like to add a junit plugin call after the parallel step has completed, I tried</p>
  
  <pre><code>stage('Unit Tests') {
        steps {
          parallel (
            "Jasmine": {
              sh 'gulp karma-tests-ci'
            },
             "Mocha": {
              sh 'gulp mocha-tests'
            }
          )
          junit 'test/coverage/junit-coverage-results.xml'
        }
      }
  </code></pre>
  
  <p>But get the error:</p>
  
  <pre><code>WorkflowScript: 18: Invalid step "parallel" used - not allowed in this context - The parallel step can only be used as the only top-level step in a stages step block
  </code></pre>
  
  <p>Is there a way I can do this before progressing to the next stage?</p>