Backdrop sends Bearer token to Stagecraft

The `/data-sets` endpoint in Stagecraft is protected by a shared bearer token.
Backdrop must send the token when it requests data set metadata from
Stagecraft

- In `BucketRepository`, send HTTP requests to Stagecraft instead of
  looking inside Mongo's `buckets` collection.
- Selectively handle HTTP 404 from Stagecraft - only `/data-sets/<name>`
  can legitimately return a 404, all other HTTP errors must bubble up.
- Fix `/data-sets/` vs `/data-sets` URL in fake Stagecraft
- Added logging on Stagecraft HTTP responses and their JSON content to
  make them easier to diagnose.
- Quite a lot of test refactoring - move unit tests that shouldn't have
  been put into integration tests for BucketRepository

Related to https://www.pivotaltracker.com/story/show/67103208
[#67103208]
Merge pull request #255 from alphagov/send-auth-token-to-stagecraft

Backdrop sends Bearer token to Stagecraft