Update subordinate design doc
    
    This commit updates the subordinate design document to reflect the current state
    and remove "outdated" message.
    
    Signed-off-by: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>