965764 - DownloaderConfig is explicit.

The DownloaderConfig used to allow setting any arbitrary config items
during init, and it would allow getting arbitrary config items even when
they had not been set. Now all possible config keys are part of its
__init__(), and it is not possible to getattr() any non-existing config
items from it.

https://bugzilla.redhat.com/show_bug.cgi?id=965764