Ansible : parameterize roles by adding variables

http://docs.ansible.com/ansible/playbooks_roles.html

- hosts: webservers
  roles:
    - common
    - { role: foo_app_instance, dir: '/opt/a',  app_port: 5000 }
    - { role: foo_app_instance, dir: '/opt/b',  app_port: 5001 }