Add option on Inputs to always set source to gl2_remote_ip

I've got the problem, that some devices send non-standard syslog formats or similar, which makes Graylog use a weird value as source (the first word of the message). It would be nice to have an option to always use the IP-address instead of anything else, similar to the option _"Force rDNS"_, since we use the IPs of the devices as identifier anyway.
## Expected Behavior

Inputs should have an option to always use the IP-address (_gl2_remote_ip_) of the incoming packet(s) for the _source_ field.

## Current Behavior

The field containing the source IP is hidden by default and uses a different name than source. Naming consistency would not be given if this field is used. Currently, all messages pass a pipeline that sets the field _source_ to the value of _gl2_remote_ip_.

## Steps to Reproduce (for bugs)

1. Setup Syslog Input
2. Send Logs with invalid header (e.g. no hostname, facility, severity and timestamp)
3. Boom, unwanted value in _source_ field.

## Context

We use IP-addresses as identifying value for our entire environment. The devices do not own DNS entries, therefor this is not an option.

## Your Environment

* Graylog Version: 2.2.2
* Elasticsearch Version: 2.4.4
* MongoDB Version: v3.4.2
* Operating System: RHEL7
* Browser version: _various_