Fixes proxy error

```
h2-proxy_1     | 2019/06/16 13:33:45 [emerg] 1#1: host not found in upstream "mercure" in /etc/nginx/conf.d/default.conf:76
h2-proxy_1     | nginx: [emerg] host not found in upstream "mercure" in /etc/nginx/conf.d/default.conf:76
sonda_h2-proxy_1 exited with code 1

```