[SPARK-20720][WEB-UI] Executor Summary' should show the exact number, 'Removed...

[SPARK-20720][WEB-UI] Executor Summary' should show the exact number, 'Removed Executors' should display the specific number, in the Application Page

## What changes were proposed in this pull request?

When the number of spark worker executors is large, if the specific number is displayed, will better help us to analyze and observe by spark ui.

Although this is a small improvement, but it is indeed very valuable.

After fix:
![executor1](https://cloud.githubusercontent.com/assets/26266482/25986597/2d8e4386-3723-11e7-9c24-e5bff17c26e2.png)

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes #17961 from guoxiaolongzte/SPARK-20720.