Use assignment_api rather than assignment

When normal dependency injection is used the variables
are named like "xxx_api".
This change makes it so that the identity backend and
drivers use "assignment_api" rather than "assignment"
so it will be easier to switch to normal dependency
injection.

Part of fix for bug 1200769

Change-Id: I7805b338c48d57ca1922bb622a3f474f2341f4ac
Merge "Use assignment_api rather than assignment"