Switch default cinder API to V2

Cinder v1 is deprecated and using v1 emits a warning (introduced in
Id8abbbb4d9b0c8c49ab51fc3d958ef0d487467f8). So update the default value
to Cinder v2.

DocImpact

Closes-Bug: #1390131

Change-Id: Ica650962c9354d741b464ec152e9e1c6d0d5fa10
Merge "Switch default cinder API to V2"