Remove detected_buildpack_guid from app api.

* First step to rethink our buildpack strategy

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Add detected_buildpack_name to apps.

* Remove buildpack_name and buildpack_guid from app model.
* Change staging task to always save buildpack guid when available.
* Do not expose detected_buildpack_name and detected_buildpack_guid to API.

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>