feat(router) Avoid nginx 502, 503, 504 errors

By default `proxy_next_upstream` only will try to use other server from upstream pool in case of `error` and `timeout`. But if for some reason the backend server fails (like if it dies) this tells nginx to try to process the request in other server.
So in case that one server fail the header `X-Deis-Upstream` will show the retries:
```
X-Deis-Upstream: 10.0.1.10:49240, 10.0.1.10:49238
```