title: ansible conditional "when" not works with role
body: <p>in my playbook i have a condition to execute role according the version in the playbook command but it executes the two roles when i add version=8</p>
  
  <pre><code>- hosts: tomcat-servers
    remote_user: root
    become: yes
    become_method: sudo
  
    pre_tasks:
      - debug: var=version
    roles:
      - { role: tomcat7, when: version == '7' }
      - { role: tomcat8, when: version == '8' }
  </code></pre>
  
  <p>the command :</p>
  
  <pre><code>ansible-playbook -i hosts site.yml --extra-vars "version=8"
  </code></pre>
  
  <p>for the debug output :</p>
  
  <pre><code>TASK [debug] *******************************************************************
  ok: [192.168.139.129] =&gt; {
      "version": "8"
  }
  </code></pre>
  
  <p>any help?</p>
  
  <p>ansible 2.1.0.0</p>