Reorganized blockstorage test packages and added V2 tests.

* Added compute integration fixtures, composites and datasets for
  blockstorage v2.
* Added images_api and attachments_api smoke tests.

Change-Id: I094ac9f59973bc2f2e3dca9d81d225f2a857a085
Merge "Reorganized blockstorage test packages and added V2 tests."