Made the repo_url a role variable

Hey Jeff,

I've been working through Ansible for Devops (love it!) and using your roles to set up a D8 blog on a webserver. Everything is working great, except that I want to be able to deploy a version of Drupal that I have on github. I see allgood2386 worked through his own implementation here: https://github.com/allgood2386/ansible-role-drupal/commit/4c7b4c80bb45607f92bcc1418d2ee5ed89373b67.

In this pull request, I think you'd have to use a public repo, but I'm also interested in how you would automate this workflow using a private repo. Maybe that's more for the book to explain, however. I think it's a common pattern, so I hope you are willing to tackle it in the book or a blog post. Thanks!