SSH: Use default_domain_suffix for users' authorized keys
    
    In commit eeecc48d22a28bb69da56f6ffd8824163fc9bf00 we disabled
    default_domain_suffix for the SSH responder, but in a wrong way -- we
    disabled the functionality completely, also for users, not only for
    computers. This might have been correct at the time, since SSH keys in ID
    overrides are a relatively new feature, but it's definitely not correct
    in general.
    
    Instead, this patch restores the use of default_domain_suffix, but only
    for looking up public keys of users, not of computers.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3259
    
    Reviewed-by: Petr Cech <pcech@redhat.com>