lighthouse only participant


So the bug happens if there is only single lighthouse node. It will be confused as it expects at least one working peer to get started. 

You can do:

```yaml
participants:
  - count: 2
additional_services:
  - dora
```

or 


```yaml
participants:
  - cl_extra_params: ["--target-peers=0"]
additional_services:
  - dora
```