Fix various PEP8 warnings
Rewrite BucketConfigRepository, talk to Stagecraft

We don't store the metadata in Backdrop any more. We're modifying
BucketConfigRepository to call the Stagecraft /data-sets endpoint to get
the metadata.
Add the STAGECRAFT_* config options to dev/test
Instantiate BucketConfigRepository with new sig
Update tests for new BucketConfigRepository