Use existing roles DRY out playbook - certificates

There are several tasks in the playbook which have been re-implemented here although they are provided by existing roles. I suggest this playbook re-uses roles which are already published, in order to reduce the size of the playbook and improve maintainability. See #7 for context

As a suggested first stab at this, the [IGTF](https://github.com/EGI-Foundation/ansible-packstack/blob/master/igtf.yaml) playbook could be replaced with [AAROC.certificates](https://github.com/AAROC/certifcates) with `needs_cert=false` or something.