sql.Driver:authenticate() signatures should match

A method called authenticate_user was renamed in
identity.backends.sql.Driver from authenticate_user
to authenticate but the base class wasn't updated.

Also, the user_id and password arguments to authenticate
should NOT be optional.

Change-Id: Ie6eb42f060e368ec99d5d8241a404cf7c70d48ae
Merge "sql.Driver:authenticate() signatures should match"