title: Jenkins Declarative Pipeline: How to read choice from input step?
body: <p>I'm trying to access a variable from an <code>input</code> step using the declarative pipelines syntax but it seems not to be available via <code>env</code> or <code>params</code>. 
  This is my stage definition:</p>
  
  <pre><code>stage('User Input') {
      steps {
          input message: 'User input required', ok: 'Release!',
              parameters: [choice(name: 'RELEASE_SCOPE', choices: 'patch\nminor\nmajor', description: 'What is the release scope?')]
          echo "env: ${env.RELEASE_SCOPE}"
          echo "params: ${params.RELEASE_SCOPE}"
      }
  }
  </code></pre>
  
  <p>Both <code>echo</code> steps print <code>null</code>. I also tried to access the variable directly but I got the following error:</p>
  
  <pre><code>groovy.lang.MissingPropertyException: No such property: RELEASE_SCOPE for class: groovy.lang.Binding
      at groovy.lang.Binding.getVariable(Binding.java:63)
      at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onGetProperty(SandboxInterceptor.java:224)
  </code></pre>
  
  <p>What is the correct way to access this choice parameter?</p>