added ansible_required to playbook and roles

This added a new parameter called 'ansible_required' to the playbook and role vars/main.yml to indicate the mininum version of Ansible this playbook or role should run on.

The way support for ansible_required for roles is implemented is pretty poor. I would like to discuss alternatives for this. Is there a better option for implementing this within the current roles mechanism. Or roles could grow something like role/info.yml which could contain ansible_required: and other role-wide parameters.