增加Arbiter
增加配置文件mongod1,mongod2,mongod3分别为shard1，shard2，shard3三个set集合的arbiter。
Note:Before running the rs.remove() operation, you must shut down the replica set member that you're removing.
Changed in version 2.2: This procedure is no longer required when using rs.remove(), but it remains good practice.
