More robust connection from nginx to upstream (uwsgi)

[palvarez89]

Some links

https://serverfault.com/questions/586125/nginx-no-live-upstreams-while-connecting-to-upstream
https://stackoverflow.com/questions/35113279/nginx-no-live-upstreams-while-connecting-to-upstream-but-upsteam-is-ok
https://stackoverflow.com/questions/13222274/uwsgicluster-no-live-upstreams-while-connec

[palvarez89]

apparently, this message is logged even though nginx tries another upstream