Default to Cinder REST API v2

Set OS_VOLUME_API_VERSION environment variable to 2 so we use
specifically Cinder REST API v2. v1 is still enabled in the catalog, but
we want more exposure to v2 for testing.

Change-Id: I6c2f29edf44a0f58a7830fe4dd2db35f2db3658c
Merge "Default to Cinder REST API v2"
Merge branch 'master' of github.com:openstack-dev/devstack