Implement OS::Cinder::VolumeAttachment

This creates a new volume attachment resource with Cinder specific
naming.

Change-Id: Ie73b929925c58e47f3509d57dd385cbd4ea5a4c1
Merge "Implement OS::Cinder::VolumeAttachment"
Fix PEP H101 (Use TODO(NAME))

Change-Id: I77c9caa650462725b7c686fbafd9753d046b138d