Add role Argument Validation

[rndmh3ro]

          With register and assert it won't work, as we need to use `ignore_errors: true` on the task. `ignore_errors: true` does not work on roles or (or the `include_role`-tasks for that matter).

What we should instead do, is use Role argument validation and there sure that this variable is set. Then we don't need additional tests for that.

If you're fine with that, I'll create an issue for it.

Originally posted by @rndmh3ro in #145 (comment)