Display warning when using deprecated kibana configs

[epixa]

We changed the configuration format in kibana.yml to use periods instead of underscores in 4.2, and while we marked the old configuration values as deprecated in the docs, we still supported them without any warning to users in the code. We should remove the deprecated configurations from the docs entirely and start triggering warnings on kibana startup whenever old configurations are used.

[jbudz]

Warning were addressed by #6825 and look like:

Docs look up to date: https://www.elastic.co/guide/en/kibana/5.0/settings.html