Figure out how to handle cloned Collector identities

When user copy the contents of `/var/lib/graylog-collector` to a different node, two nodes will share the same identity. The same happens if the Collector is enrolled and the disk is used as an image (e.g., AWS AMI) to start multiple other nodes.

We need to figure out how we can detect cloned identities and if we can prevent it.

Ideas:

- Use a machine ID as instance UID (might be problematic if that machine ID changes)