Fix bad attribute name in get_auth_backend.
Fix bad attribute name in get_auth_backend.

The attribute name used for the backend ID in the auth backends registry
was incorrect, preventing this method from ever working. This change
fixes it.

Testing done:
Used get_auth_backend successfully.

Reviewed at https://reviews.reviewboard.org/r/10092/

chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard