cache_req: Don't force a fqname for files provider output
    
    As we're enforcing the output of files provider to be fully-qualified we
    can face some weirdness when using domain_resolution_order as:
    [user@implicit_files@machine]$
    
    This is not only not coherent but also causes issues when the local
    user, which is managed by the files provider, tries to do a `sudo su`.
    
    As the most common scenario for *local* users is to have the user
    (non-fully-qualified) in sudoers and, as sudo simply compares usernames,
    changing the output from non fully-qualified to fully-qualified would
    break this scenario, not allowing the user which has sudo access to use
    sudo.
    
    In order to avoid the issues described above, let's just not force the
    output of the files provider to be fully-qualified.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3743
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>