"Shadow users: unified identity" implementation:
Separated user identities from their locally managed credentials by
refactoring the user table into user, local_user, and password tables.
user -> local_user -> password
-> federated_user
-> ...
(identity) -> (credentials)
Migrated data from the user table to the local_user and password tables.
Modify backend code to utilize the new tables.
Note: #2 "Shadow LDAP and federated users" will be completed in a
different patch. The federated_user table will be added with that patch.
bp shadow-users
Change-Id: I0b6c188824e856d788fe7156e4a9dc2a04cdb6f8