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

It's to warn people who got it enabled without realizing it, since platformstack wasn't included. I felt that the 'implicit-enabled' logic deserved notifying the user.