proxy: increase timeout since raven can take lots of time to process …

…requests

Fix this timeout error in some Raven notebooks:
```
HTTPError: 504 Server Error: Gateway Time-out for url:
https://pavics.ouranos.ca/twitcher/ows/proxy/raven/wps
```

This is just a work-around since something is very wrong on our
production host Boreas (a physical host with 128G ram and 48 logical cpu).

During the test, Boreas was having this "load average: 6.35, 5.90,
4.33".  For its hardware specs, it is basically idle.

The increased timeout was not needed for my test VM (10G ram, 2 cpu),
medus.ouranos.ca (physical host with 16G ram, 16 logical cpu) and
hirondelle.crim.ca (VM with 32G ram, 8 cpu).

birdhouse/config/proxy/nginx.conf

@@ -31,7 +31,7 @@ http {
     client_body_timeout 600s;
 
     # timeout for reading a response from the proxied server
-    proxy_read_timeout 120s;  # default 60s
+    proxy_read_timeout 240s;  # default 60s
 
     include /etc/nginx/conf.d/*.conf;