Graylog Outputs with HTTP_Proxy

Graylog Outputs not working via http_proxy_uri

## Current Behavior

Hi,

We have a **Server A** where Graylog is installed with a **Stream X**
And Another **Server B** where Graylog is installed with a **Stream Y**

Data is coming into X via a Beats inputs (**http://ServerA:5044**). A GELF HTTP input (**http://ServerB:12201**) is running on B.

**Server A** has Internet connectivity via an http_proxy (**http://proxyserver:80**).
There are environment variables defined at the Shell level on A
**export http_proxy=http://proxyserver:80**

We wrote a sample **nodejs** script which pushes a test message to **http://ServerB:12201** and executed on Server A. We can see the data on the Server B.
This confirms that the GELF endpoint is **reachable** via the Proxy Server and the ports are open.

Now, we have configured GELF OUTPUT for Stream X and push data to Server B at **http://ServerB:12201**
We then configured the below parameter in Server.conf and restarted Graylog

> http_proxy_uri = http://proxyserver:80

But now, we see errors in the logs like

> "unable to connect to ServerB"

Below is the configuration for the output:

```
Gelf ID: 5a473a67644f1814953f4d19
Type: org.graylog2.outputs.GelfOutput
connect_timeout: 1000
hostname: ServerB
max_inflight_sends: 512
port: 12201
protocol: TCP
queue_size: 512
reconnect_delay: 500
tcp_keep_alive: false
tcp_no_delay: false
tls_trust_cert_chain:
tls_verification_enabled: false

```
Please suggest what should we try to solve this.

Thanks

## Steps to Reproduce (for bugs)

We tested the above steps in our environment.

## Context

We want to push data from one Stream to another Stream on another Graylog instance.

## Your Environment

* Graylog Version: 2.3.1
* Elasticsearch Version: 5.5
* MongoDB Version: 3
* Operating System: Ubuntu 16
* Browser version: Chrome