More apt repo improvements

* Adding a snippet to rewrite sources.list for debian/ubuntu
  so that there's a nice clean file without a lot of extraneous
  entries, plus a fix to the preseed_apt_repo_config for repos that
  are not mirrored locally (incorrect url formatting)
* Moving the repo_data logic into the utils.blender function
  This makes the repo data available to any template function that
  calls blender (which is most of them) as opposed to having to copy
  the logic into each individual function