Add volume backup/restore integration test

Adds a more comprehensive test for the cinder volume resources:
- Creates a stack with a volume, and writes data to it
- Deletes the stack with the volume deletion policy set to
  "snapshot" (which really means backup) the volume
- Create a new stack with a volume created from the backup
- Prove the data written in the first stack is still present
Note this test also aims to provide coverage of volume attachment
resources, e.g so we would catch any bugs like bug #1311533 in
future.

Authored-By: Steve Hardy <shardy@redhat.com> based on tempest change
I04ae0cf942d12c4504b2df504a8c940575b90b69

Change-Id: I04ae0cf942d12c4504b2df504a8c940575b90b69
Related-Bug: #1311533
Merge "Add volume backup/restore integration test"