[Kibana] Missing "basepath" variable in Kibana integration settings

[tonyghiani]

📓 Summary

The Kibana integration documentation states

If the Kibana instance is using a basepath in its URL, you must set the basepath setting for this integration with the same value.

Even if the parameter is already defined as a metricbeat configuration in the data streams stream.yml.hbs file,
when accessing the settings configuration on the Kibana UI, the basepath variable is not available for configuration.

To fix this visibility issue and make the basepath configurable, we should update the integration manifest to include this field in the vars block.

✔️ Acceptance criteria

• The basepath parameter should be defined under the integration's manifest vars block
• The basepath setting should be available in the configuration panel of Kibana integration.

[elasticmachine]

Package kibana - 2.3.6 containing this change is available at https://epr.elastic.co/search?package=kibana