Docs build fail due to deprecated config option in Castellan

[Sean McGinnis]

With the raise of g-r for Castellan to 0.14.0, our docs builds have now started failing with the error:

Warning, treated as error:
Config option <oslo_config.cfg.OptGroup object at 0x7f1da1360790>.api_class is deprecated. Use option key_manager.backend instead.

This is due to commit:

https://github.com/openstack/castellan/commit/8980bf7da55dd084ad84c84534fe937f0d43b9c0#diff-fb732054a24cd0443791b7696cb79a3fR24

We need to handle switching from key_manager.api_class to key_manager.backend.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/505752
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=e75be5d90519094fca3ee475b906e7c2fe1d09fd
Submitter: Jenkins
Branch: master

commit e75be5d90519094fca3ee475b906e7c2fe1d09fd
Author: Sean McGinnis <email address hidden>
Date: Wed Sep 20 12:12:25 2017 -0500

    Switch to use key_manager.backend

    Castellan commit 8980bf7da55dd084ad84c84534fe937f0d43b9c0 deprecated the
    api_class option in favor of the new backend option. This causes doc build
    failures in Cinder due to us treating warnings as errors.

    This changes over to use backend (the library will still recognize api_class
    as an option) and removed some of the key manager code deprecated in Newton.

    Depends-on: I112f7a4654a65c3291526b408838d7b1c175b069
    Closes-bug: #1718468
    Change-Id: I8d3638a69f4efcc52053269c51c01667fb810172

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/cinder 12.0.0.0b1 development milestone.