Currently ec2api uses the volume v2 API by default, but v2 API was
deprecated a while ago and will be removed fron cinder shortly.
This patch ensures that ec2api uses the volume v3 API instead by
default.
Closes-Bug: #1908993
Change-Id: I280d3c009893c67d215b0c7106eec7fe2435c335
eb3b4851a9