Correct inheritance of nova.volume.cinder.API

nova.volume.cinder.API was deriving incorrectly from nova.db.base. This
patch simply removes that unnecessary import and changes
nova.volume.cinder.API to derive from object instead.

Change-Id: I5092b7fbb3780317a09a512aed7ecbe1d57a4428
Closes-bug: #1292997
Merge "Correct inheritance of nova.volume.cinder.API"