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
Beef up testing, additional chef-solo support

- 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
Extra blank line, rubocop
Merge pull request #79 from martinb3/master

Better elkstack agent handling