ccloudvm download manager improvements

[markdryan]

A whole pile of work needs doing to the download manager

1. It should be possible to enumerated cached files.
2. It should be possible to delete cached files.
3. It should be possible to refresh a downloaded file.
4. The key used to access each downloaded file is the file's name. This needs to change. It should be the URL, otherwise we cannot support two files with the same name downloaded from different URLs.
5. The cache folder shouldn't be called .cache as it is not a cache. It cannot be deleted without breaking existing images.
6. Consider location of cached files.