MXS-1220: Respond with 200 OK to root level requests

If a request to the `/` resource is made, the API responds with an 200
OK. This is done to make it possible to use the HTTP health check
mechanism found in many cloud load balancers.

server/core/admin.cc

@@ -104,7 +104,12 @@ int Client::process(string url, string method, const char* upload_data, size_t *
     HttpRequest request(m_connection, url, method, json);
     HttpResponse reply(MHD_HTTP_NOT_FOUND);
 
-    if (request.validate_api_version())
+    if (url == "/")
+    {
+        // Respond to pings with 200 OK
+        reply = HttpResponse(MHD_HTTP_OK);
+    }
+    else if (request.validate_api_version())
     {
         reply = resource_handle_request(request);
     }