Add precise to releases, and disk.img artifacts
Create symlinks for ubuntu-cloud releases

* Move remaining download logic into separate method

* Add method to check for, add, and update 'current' symlinks for each
  release to point to the latest version, as well as link the version
  directory to the release directory

* Minimize output except when downloading, creating a new symlink,
  or retargeting an existing symlink

* Simplify nested loops into comprehensions

* Download in ascending order rather than descending