Move _plugin api to _node/plugin + add tests

Fixes #5381

logstash-core/lib/logstash/api/rack_app.rb

@@ -104,7 +104,7 @@ module LogStash
           "/_node" => LogStash::Api::Modules::Node,
           "/_stats" => LogStash::Api::Modules::Stats,
           "/_node/stats" => LogStash::Api::Modules::NodeStats,
-          "/_plugins" => LogStash::Api::Modules::Plugins
+          "/_node/plugins" => LogStash::Api::Modules::Plugins
         }
       end
     end

logstash-core/spec/api/lib/api/node_plugins_spec.rb

@@ -0,0 +1,32 @@
+# encoding: utf-8
+require_relative "../../spec_helper"
+require "sinatra"
+require "logstash/api/modules/plugins"
+require "logstash/json"
+
+describe LogStash::Api::Modules::Plugins do
+  include_context "api setup"
+
+  extend ResourceDSLMethods
+
+  before(:all) do
+    do_request { get "/" }
+  end
+
+  let(:payload) { LogStash::Json.load(last_response.body) }
+
+  describe "retrieving plugins" do
+    it "should return OK" do
+      expect(last_response).to be_ok
+    end
+
+    it "should return a list of plugins" do      
+      expect(payload["plugins"]).to be_a(Array)
+    end
+
+    it "should return the total number of plugins" do
+      expect(payload["total"]).to be_a(Numeric)
+    end
+    
+  end
+end