Make id parameter optional in logstash.get_pipeline API

rest-api-spec/src/main/resources/rest-api-spec/api/logstash.get_pipeline.json

@@ -11,6 +11,10 @@
     },
     "url":{
       "paths":[
+        {
+          "path":"/_logstash/pipeline",
+          "methods":[ "GET" ]
+        },
         {
           "path":"/_logstash/pipeline/{id}",
           "methods":[ "GET" ],