NSS: check for overrides before calling backend
    
    Currently the flag that the input data in a user or group lookup request
    might be an override value is only set if no cached entry was found. If
    the cached entry of an object with overrides is expired and a request
    with the override value as input is processed the flag is not set and
    the backend might not be able to find the right entry on the server.
    Typically this should not happen because of mid-point refreshes. To
    reproduce this create a FreeIPA user and override the login name for a
    specific view. On a client which has this view applied call
    
    getent passwd overridename
    sss_cache -E
    getent passwd overridename
    
    The second getent command will still show the right output but in the
    logs a
    
    [sss_dp_get_reply] (0x1000): Got reply from Data Provider - DP error
     code: 3 errno: 0 error message: Account info lookup failed
    
    message can be found for the second request.
    
    Related to https://fedorahosted.org/sssd/ticket/2642
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 35b178d02dfd293778aefbc0b465a5a3a4b6cd8f)