Setting the leader in the Configuration in the Server class at runtime

## Current Behavior

https://github.com/Graylog2/graylog2-server/blob/596b03094601826e7c1430bbcebe0a7bb3472073/graylog2-server/src/main/java/org/graylog2/commands/Server.java#L278

changes the configuration at Runtime. I think there are two problems with this:
- first, I think the Configuration classes should be immutable
- second, this change in configuration clashes with the current way of leader-election via the service which, IMHO should only be derived from runtime state in the Leader Election classes (which was probably initialised from the configuration on startup)