Generated templates don't include fields when parent defined in 2 locations

[exekias]

Some processors share fields with metricbeat modules (docker & kubernetes), so they define some fields like this:

https://github.com/elastic/beats/blob/master/libbeat/processors/add_docker_metadata/_meta/fields.yml

While the rest of fields for the module are in other fields.yml file:

https://github.com/elastic/beats/blob/master/metricbeat/module/docker/container/_meta/fields.yml

As they share the same root key, the first one in the resulting fields.yml is overridden by the second one. In practice, this means that fields from docker & kubernetes processors are missing from the final template