Support for 'default' bind_address in global input configuration

## Expected Behavior
Being able to leave `bind_address` field to the default value from configuration file on global input creation. With this change, every node in the cluster should pick correct IP address.

## Current Behavior
Right now you only have two options on global input creation: `127.0.0.1` and `0.0.0.0` addresses which are not ideal if you just want to accept connections on single no-loopback interface.

## Possible Solution
Accept default value for `bind_address` from `server.conf`.

## Context
We have graylog cluster with 3 nodes, each one binds to it's primary interface IP address:
`10.0.0.1, 10.0.0.2, 10.0.0.3`
I want to create a global input that binds only to these IP's.

* Graylog Version: 2.3.2
* Elasticsearch Version: 5.6.4
* MongoDB Version: 3.4.10
* Operating System: Ubuntu 16.04.3 LTS