Fix assignment to not require user or group existence

The assignment code was requiring the existence of users and
groups before performing different operations. With federation,
the users and groups may legitimately not be defined in keystone,
so these checks cannot be done.

Implements: bp no-check-id
Change-Id: Ib9d0188ae2a436617bad90bf936e7b3dbf296885
Merge "Fix assignment to not require user or group existence"