[cli] properly rewrite download-build to use APIv3
    
    The download-build was reimplemented to APIv3 some time ago,
    but with mistakes. The command wasn't working at all. This
    commits finishes it.
    
    There are also changes to tests related to download-build command.
    It was necessary to use `configure_mock`, because we needed to mock
    a `name` value, which is unfortunately `MagicMock`'s property used
    for string representation of such object.