Add info about buildpack used to staging response

The goal is to get information about the buildpack used for staging (including detected admin buildpacks) back to the CC so it can be used to populate app usage events.  This intended to be part of the resolution to cloudfoundry/cloud_controller_ng#178.

Highlights:
* Add the path of buildpack used for staging to staging_info.yml so the buildpack manager can be used to detect its type
* Refactor buildpack manager out of the staging workspace
  - The staging task now creates and holds a reference to the buildpack manager
  - The buildpack manager is passed to workspace prepare so prepare can get the list of buildpack directories
  - The buildpack ordering tests were consolidated in the buildpack manager specs
* Added methods to buildpack manager to return the buildpack key or a system buildpack url from a path
* Return the buildpack url (system or custom) and the buildpack key in the staging complete message

I'm not particularly happy with the names `buildpack_url` and `buildpack_key` in the response so if there are better suggestions, please let me know.

As always, this is a proposal so I'm happy to work out the details.

/cc @markkropf @fraenkel