Add fields to `.logstash`'s mapping in template (elastic/x-pack-elasticsearch#3872)

* Add fields to `.logstash`'s mapping in template

This "makes room" in the index for pipeline settings and node groups. Due to this change, users will be able to specify settings and node groups for a pipeline via the Centralized Config Management UI in Kibana. Logstash will only retrieve pipelines associated with the node group specified via the `xpack.management.group.id` setting in `logstash.yml`. For the retrieved pipelines, Logstash will apply any (optionally) specified pipeline settings before (re)loading the pipelines.

* Making field name more explicit + adding multi field for better search

Original commit: elastic/x-pack-elasticsearch@2df101f0b1

plugin/core/src/main/resources/logstash-index-template.json

@@ -33,6 +33,19 @@
         "pipeline":{
           "type":"text"
         },
+        "pipeline_settings": {
+          "dynamic": false,
+          "type": "object"
+        },
+        "pipeline_node_group_ids": {
+          "type": "keyword",
+          "fields": {
+            "searchable": {
+              "type": "text",
+              "analyzer": "simple"
+            }
+          }
+        },
         "username":{
           "type":"keyword"
         },