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
Feature tests working with fake Stagecraft server

Backdrop now gets its config from Stagecraft rather than storing it
itself. We need a fake Stagecraft server to return config for feature
tests where we cannot stub out the Stagecraft client interface.
Removed unused method and added empty arg error handling
Merge pull request #252 from alphagov/stagecraft-integration

Stagecraft integration