Support Cinder API version 2

Add support for the second version of the Cinder API, which brings
useful improvements and will soon replace version one.

Add a new 'cinder_api_version' option in sahara.conf.

Implements: blueprint support-cinder-api-v2
Change-Id: I57eddf3cf59e8b6b087e2ff9bfcd002175d3b1f9
Merge "Support Cinder API version 2"