cinder manage/unmanage scenario test

This test will cover the Manage/Unmanage feature of cinder. The manage feature consists of getting a volume - present at the backend but not registered at Cinder - and start to manage it (registering it in Cinder, and accessing it in the backend). The unmanage feature consists of the exactly reversed action, meaning a volume - present in Cinder and backend - will no longer be managed by Cinder.