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

That's the status, but what's the implication you are trying to make here?

'and it should not have been enabled' for instance