增加Arbiter

增加配置文件mongod1,mongod2,mongod3分別為shard1，shard2，shard3三個(gè)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.