Blockstorage and volumes_api test update

 * Moved dataset generators to cloudcafe
 * Updated fixtures with custom assertions for volumes testing
 * Tests are now data-driven as a rule, with few exceptions.
 * Snapshot tests now support volume-type restoration
   configuration.

Change-Id: I2287e059eb3f6e4aa64e02bf8531d9cfd8bc700c
Merge "Blockstorage and volumes_api test update"