keymgr_api_class is now deprecated in favor of keymgr_backend, with
full backward compatibility for users.
keymgr_backend is set to cinder.keymgr.conf_key_mgr.ConfKeyManager
by default and configure key_manager/backend instead of
key_manager/api_class. It reflects what openstack/cinder did in Queens
cycle.
Change-Id: I6f015a4ed4980d37622bb2931477af916cfc9003
Related-Bug: #1725298