Add support for tests from tar and url
Since LAVA is not designed as a build farm, it will be used to work with various build farm together. So, it will be useful to support tests/images from different sources.
Currently, LAVA supports tests from git/bzr and inline. Support for tar and url seems to be not usable yet. So, what is the expected way to do that? Reuse DownloaderAction or something else?