Allow custom configuration for Elasticsearch databases

[wallrj]

/kind feature

Users of Navigator have unique requirements and want to supply their own Elasticsearch configuration options
But Navigator requires particular database configuration in order to safely perform its operations.

One proposal is to provide a whitelist of configuration options that can be safely tweaked by the user.

The user then supplies a configuration file / blob which gets merged with the navigator configuration before the database process is (re)loaded.

See #251