https://review.openstack.org/#/c/595372 removed the capability to use
the loading by module name and requires that the backup driver class name
is provided.
This fix updates the defaults to use the full class name, for example
``cinder.backup.drivers.swift.SwiftBackupDriver``.
Change-Id: I188d9383937a8425a78611eb168d55264b7b5c9f