Remove a useless arg in range()

Modify tests/test_backend.py, remove the useless arg ("start index" = 0)
since its default value is 0.

Change-Id: Ia88c538a6dfe751e94fdb7f465ab87ce72cfa108
assignment backend

Splits the assignments functions off of the identity api
and manager, and moved them into their own backend.

To prevent breaking existing code, this adds assignment delegation
functions to Identity Manager.

There is a circular dependency between ID and assignments.

This code is mostly pure refactoring, with no changes to the
unit tests.  Existing behavior is maintained.

In the future, we will add unit tests for mixing an LDAP
identity provider with a SQL assignment backend.

blueprint split-identity

Change-Id: I6c180aa1ae626ace5b91e0bf1931bdaf2aa031d5
Merge "Remove a useless arg in range()"