Collector Configuration missing Log Collector Type when exported in Content Pack

The definition of a Collector Configuration has a "Unknown collector" type when it's imported back from a Content Pack.

## Steps to Reproduce (for bugs)
1. In Sidecar>Collector Configuration, define a configuration with Collector set to filebeat
2. In Content Pack, create a new one (be sure the above Collector Configuration is selected) and download it
3. In a pristine Graylog instance, Upload and apply the above Content Pack

* Graylog Version: 3.2.6