Motivation/summary

When configuring an Elasticsearch instance for API Key usage, Elasticsearch default values should be merged with the custom configuration.

At the moment default values are not merged as soon as any of the apm-server.api_key.elasticsearch.* options is configured, which is unexpected behavior and not aligned with how other configuration options are merged with default values.

This PR changes the behavior to use default options where no dedicated configuration option is provided. This change also allows to add the required flag for the Elasticsearch host option again, as now the host is always set by default.

Checklist

- [ ] I have signed the Contributor License Agreement.

• My code follows the style guidelines of this project (run make check-full for static code checks and linting)
• I have rebased my changes on top of the latest master branch

- [ ] I have made corresponding changes to the documentation

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

• I have updated CHANGELOG.asciidoc

How to test these changes

Configure some option of apm-server.api_key.elasticsearch.* and ensure default values are applied for missing configuration options.
Default values are hosts: []string{"localhost:9200"}, protocol: "http", timeout: 5s.

E.g. ensure to have a locally running Elasticsearch instance with security enabled and run ./apm-server apikey create -e -E apm-server.api_key.enabled=true -E apm-server.api_key.elasticsearch.username=foo -E apm-server.api_key.elasticsearch.password=bar
When providing proper username+password this should create an API Key, by using the default host configuration of localhost:9200.

Related issues

fixes #3341