[feature request] Add backup server to upstream via UI

Hi,

In nginx you can configure a backup server to the upstream directive:

1upstream example-cache {
2    server 10.0.0.1:8080 max_fails=0;
3
4    server 10.0.0.2:80 backup max_fails=0;
5
6    keepalive 10;
7}

Can this feature be added to the UI?

In our case the upstreams are different varnish nodes. If these are dead, the application behind couldn't be reached even if it is still healthy