Code change for nova support cinder client v2

Use v2 volume endpoint to attach/detach volume would be failed,
due to nova not supporting cinder client v2 yet. This patch is for
nova support cinder client v2.

Implements bp support-cinderclient-v2
Closes-Bug: #1215772
Co-Authored-By: Mike Perez <thingee@gmail.com>
Co-Authored-By: Yaguang Tang <yaguang.tang@canonical.com>

Change-Id: Id8abbbb4d9b0c8c49ab51fc3d958ef0d487467f8