kubernetes provider (#772)

* feat: Kubernetes ServiceDiscoveryProvider

* 编写k8s测试例子

* feat：fix kube config

* feat： remove port

* feat ： complete the k8s test

* feat :  add kubeserviceDiscovery test

* feat : add kube provider unittest

* feat :add kubetnetes docs

how to use ocelot with kubetnetes docs

* keep the configuration as simple as possible, no qos, no cache

* fix: use http

* add PollingKubeServiceDiscovery

* feat : refactor logger

* feat : add  pollkube docs

* feat：Remove unnecessary code

* feat ： code-block json

samples/OelotKube/DownstreamService/Program.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Logging;
+
+namespace DownstreamService
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            CreateWebHostBuilder(args).Build().Run();
+        }
+
+        public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
+            WebHost.CreateDefaultBuilder(args)
+                .UseStartup<Startup>();
+    }
+}