Better elkstack agent handling

- Exit the agent recipe if no servers are found, don't try to proceed
- Add warnings about what the discovery logic is doing for the agent
- Don't search for server/cluster nodes on chef-solo, allow overrides
- Fixup tests to handle the chef-solo case but beef up check logic for fall through

I wanted to write logs for the two different cases, but really it should be one conditional:
```
logging_enabled = enable_attr.nil? || ( !enable_attr.nil? && enable_attr )
```