Change Cinder api_class setting to backend

Castellan switched the `api_class` config option to `backend` in commit
8980bf7da55dd084ad84c84534fe937f0d43b9c0. The old setting will still be
recognized for now, but we should switch to using the new, correct config
option.

Change-Id: I5e46c738531d5d56777e91a00f4cee9531356f2e
1 file changed