support master/standby

#master is sigle
upstream search_service{
    server 127.0.0.1:8983;
    server 127.0.0.1:8984 weight=5 backup#standby;
    server 127.0.0.1:8985 weight=3 backup#standby;
    server 127.0.0.1:8986 weight=1 backup#standby;
}