Update nova api_class setting to backend

Castellan switches the `api_class` config option to `backend`. The change
is still backwards compatible with the old `api_class` setting, but cinder
already updated to use the new option (see I5e46c738531d5d56777e91a00f4cee9531356f2e)
and it is better to use the new setting.

Change-Id: Ib609c82e7076d19676baaf4f08abd79ea11db0e3
1 file changed