Changing openrc default Cinder API version to v3

* v3 is a superset of v2 and has been the defacto Cinder version for
several years now.
* Devstack installs Cinder v3 API by default, so the default environment
variables should reflect this.

Change-Id: I86e1ae4e020e2be043cf8e190d7959b65b6c093c
1 file changed