Moved RBDDriver, SheepdogDriver into their own respective driver
files under cinder/volume/drivers/*. Moved FakeISCSIDriver and
LoggingVolumeDriver into a fake driver file under the tests,
since that's the only place where those are used.
Implements bp driver-cleanup
Change-Id: I39d91ac8e498a9e42c5443db16706203b988444b