title: How to skip role executing in Ansible
body: <p>I try to write the playbook.yml for my vagrant machine and I'm faced with the following problem.
  Ansible prompt me to set these variables and I set these variables to null/false/no/[just enter], but the roles is executed no matter! How can I prevent this behavior? I just want no actions if no vars are set..</p>
  
  <pre><code>---
  - name: Deploy Webserver
    hosts: webservers
    vars_prompt:
      run_common: "Run common tasks?"
      run_wordpress: "Run Wordpress tasks?"
      run_yii: "Run Yii tasks?"
      run_mariadb: "Run MariaDB tasks?"
      run_nginx: "Run Nginx tasks?"
      run_php5: "Run PHP5 tasks?"
  
    roles:
      - { role: common, when: run_common is defined }
      - { role: mariadb, when: run_mariadb is defined }
      - { role: wordpress, when: run_wordpress is defined }
      - { role: yii, when: run_yii is defined }
      - { role: nginx, when: run_nginx is defined }
      - { role: php5, when: run_php5 is defined }
  </code></pre>