Volume API v2 has been deprecated for a long time.
There is no reason to use volume v2 in clouds.yaml by default.
This commit also drops "--os-identity-api-version 3" from
write_clouds_yaml in functions -common as "3" is the default value
of tools/update_clouds_yaml.py. They are hardcoded in DevStack
so there is no reason to pass it.
Change-Id: Ie84026a3d19f7711fc781b7012355096c7ff6b5a