Provide a path for default xml to load upon creation.

I had to get a bit more clever here and abstract away getting the xml content's data via a function and make sure it was consistently used.
@Kelketek I see, good point. In the case of mentoring, access to the parameters isn't strictly necessary though I think, you could get rid of the need to access `self.default_xml_content`. If you look at https://github.com/edx-solutions/xblock-mentoring/blob/b59e5e4fb0d75c8a18da0b1a687c674d2e527f50/mentoring/mentoring.py#L423 you'll see that all it does is use `self.url_name_with_default`, which is just a random id, and include that in the template: https://github.com/edx-solutions/xblock-mentoring/blob/master/mentoring/templates/xml/mentoring_default.xml#L1 . That could be factored out to a function which could be used as the callable default, prevent the need to break abstraction layers, and allow to reuse it on the other xblocks.