Implement pipeline conditional match in output.elasticsearch

[cristianocasella]

Like described here:

https://www.elastic.co/guide/en/beats/filebeat/current/elasticsearch-output.html#_pipelines

Filebeat support the ability to split in different pipelines depending, as a sample, to a field match (field 'type' in the example).

In your module this condition is not supported.

From templates/filebeat5.yml.erb (@103-105):

 <%- if @filebeat_config['output']['elasticsearch']['pipeline'] != nil -%>
  pipeline: "<%= @filebeat_config['output']['elasticsearch']['pipeline'] %>"
  <%- end -%>

the support for an array 'pipelines' containing for each pipeline a condition could be implemented.