Use volume v3 API in clouds.yaml by default

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
diff --git a/tools/update_clouds_yaml.py b/tools/update_clouds_yaml.py
index eb7265f..3aad0e0 100755
--- a/tools/update_clouds_yaml.py
+++ b/tools/update_clouds_yaml.py
@@ -83,7 +83,7 @@
     parser.add_argument('--os-cloud', required=True)
     parser.add_argument('--os-region-name', default='RegionOne')
     parser.add_argument('--os-identity-api-version', default='3')
-    parser.add_argument('--os-volume-api-version', default='2')
+    parser.add_argument('--os-volume-api-version', default='3')
     parser.add_argument('--os-cacert')
     parser.add_argument('--os-auth-url', required=True)
     parser.add_argument('--os-username', required=True)