[FLINK-2769] [runtime-web] Set allow-origin header

Many browsers don't allow cross-origin HTTP requests if the respective
HTTP header is not set by the server.

Because of this it was not possible to test changes to the web frontend
with the local proxy server and a running job manager.

See here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

flink-runtime-web/src/main/java/org/apache/flink/runtime/webmonitor/RuntimeMonitorHandler.java

@@ -119,6 +119,7 @@ public class RuntimeMonitorHandler extends SimpleChannelInboundHandler<Routed> {
 			response = new DefaultFullHttpResponse(
 					HttpVersion.HTTP_1_1, HttpResponseStatus.OK, Unpooled.wrappedBuffer(bytes));
 
+			response.headers().set(HttpHeaders.Names.ACCESS_CONTROL_ALLOW_ORIGIN, "*");
 			response.headers().set(HttpHeaders.Names.CONTENT_TYPE, contentType);
 		}
 		catch (NotFoundException e) {