NSS: Do not check local users with disabled local_negative_timeout
    
    sssd_nss can set different negative timeout for local users
    and groups. However, checking whether user/group is local
    is quite expensive operation. We can avoid such operations
    if local_negative_timeout is not set.
    
    This fix improve performance(40%) of lookup non-existing
    entries in offline mode and with disabled local_negative_timeout.
    
      sh$ cat pok.sh
      for i in {1..10000}; do
        getent passwd -s sss temp$i
        getent group -s sss temp$i
      done
    
      #without patch
      sh $time /bin/bash pok.sh
      real    0m41.534s
      user    0m3.580s
      sys     0m14.202s
    
      #with patch
      sh $time /bin/bash pok.sh
      real    0m26.686s
      user    0m3.292s
      sys     0m13.165s
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3122
    
    Reviewed-by: Petr Cech <pcech@redhat.com>