gistfile1.yml

---
- hosts: all
  roles:
    - packages
    - role: nginx
      when: inventory_hostname in groups['webservers']
    - role: moin
      when: inventory_hostname in groups['wikiservers']

---
- host: webservers:wikiservers
  roles:
    - role: supervisor

- hosts: webservers
  roles:
    - role: nginx

- hosts: wikiservers
  roles:
    - role: moin