Use identity auth v3 the README examples

Identity auth v3 is the default in devstack. The keystone team
advertises all deployments to migrate over to v3.

If we get our examples to use v3, that would be a great help.

Change-Id: I8bd4cbf16cd42fa1654776f19bf113e3c94e25cf

Author: samueldmq

README.rst

@@ -84,15 +84,21 @@ options as listed in  http://docs.openstack.org/developer/python-openstackclient
 Authentication using username/password is most commonly used::
 
    export OS_AUTH_URL=<url-to-openstack-identity>
+   export OS_IDENTITY_API_VERSION=3
    export OS_PROJECT_NAME=<project-name>
+   export OS_PROJECT_DOMAIN_NAME=<project-domain-name>
    export OS_USERNAME=<username>
+   export OS_USER_DOMAIN_NAME=<user-domain-name>
    export OS_PASSWORD=<password>  # (optional)
 
 The corresponding command-line options look very similar::
 
    --os-auth-url <url>
+   --os-identity-api-version 3
    --os-project-name <project-name>
+   --os-project-domain-name <project-domain-name>
    --os-username <username>
+   --os-user-domain-name <user-domain-name>
    [--os-password <password>]
 
 If a password is not provided above (in plaintext), you will be interactively