runner + callbacks: allow callbacks to template the delegates

Testing the latest 1.7 devel, I noticed the new feature where delegates are mentioned via the playbook callbacks. Given I often use templates/variables in `delegate_to:` I noticed that wasn't being templated in the callback, which has no access to host variables.

I converted the creation of the runner inject to a method, so callbacks can retrieve it and do the necessary templating, as discussed with @jimi-c.