API version not automatically updated to v3 when upgrading to Queens

The keystone charm config says that:

  preferred-api-version:
    description: |
      Use this keystone api version for keystone endpoints and advertise this
      version to identity client charms. For OpenStack releases < Queens this
      option defaults to 2; for Queens or later it defaults to 3.

However, when upgrading from Pike to Queens the api_version relation data key does not get updated:

$ juju run -u keystone/1 -- relation-get -r81 api_version keystone/1
2

This causes inconsistencies in template rendering. For example, the [keystone_authtoken] section in glance-api.conf is rendered with correct auth_uri, but empty domain names:

[keystone_authtoken]
auth_type = password
auth_uri = http://<keystone vip>:5000/v3
auth_url = http://<keystone vip>:35357/v3
project_domain_name =
user_domain_name =
project_name = services

This breaks the glance API, and causes (for example) image listing to fail.

The workaround is to explicitly set preferred-api-version to 3. It can then be reset to undefined, api_version will stay at 3.