Add support for testing distribution / sites

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So I spent a large amount of time making two distributions work with Drupal TI as it is currently, which I was able to do by overriding the 'before_script' and 'script' steps. Hopefully some of what I've done can be used toward this, but the main issue I can foresee is that the method of downloading core I've used isn't exactly standardized.

You can see one of the commits here: https://github.com/drupalmel/drupalcampmel/commit/1f0bc1417a07e3e9c252342807edbdeafea4873d

I intend to spend a bit more time on this in the future, because it could definitely be easier than the approach I had to take.