We should align with ES, Beats & LS here... use lower case with _ as a separator.

For example, instead of kibana.defaultAppId use kibana.default_app_id (actually, the in this specific example, I'd even remove the _id part... kibana.default_app is enough)

/cc @epixa

Update 2019-12-05:
Until we've refactored all config keys in a single big refactor, all existing and new config keys should keep using camelCase for consistency.