kibana.yml should use dot-notated keys

[spalger]

With the new server, config is more extensible to support plugin development. To accomplish this, we are moving to dot-notated config keys. The kibana.yml file should be updated to use dot-notation, but still support the legacy underscored_key_format for any known keys.

• support defining arbitrary config in the file
• validate config against plugins' config schemas
• update config references in the docs

known keys: https://github.com/simianhacker/kibana/blob/048cd0e2933ffc8b432505fb4fff1648cff5647d/src/server/lib/load_settings_from_yaml.js