Use current Cinder driver path

The Cinder driver path changed from `cinder.volume.X` to `cinder.volume.drivers.X`. Use of the old path results in deprecation warnings in the cinder-volume logs, ex:

>  WARNING [cinder.volume.manager] Driver path cinder.volume.solidfire.SolidFire is deprecated, update your configuration to the new path.
Merge pull request #46 from DavidWittman/patch-1

Use correct Cinder driver path