【Advice】When the number of BE is less than 3, default replica_num should be setted to the number of BE

[xinghuayu007]

Describe the bug
When the number of BE is less than 3, create a table will set default replica_num to 3, which will cause creating a table failed. It is better to set default replica_num to the numbe of alive BE, when the numbe of BE is less than 3 and the param "replica_num" is not setted。

To Reproduce
Steps to reproduce the behavior:

1. only add one BE to the system
2. create a table, replica_num is not setted:

CREATE TABLE table4 ( siteid INT DEFAULT '10', citycode SMALLINT, username VARCHAR(32) DEFAULT '', pv BIGINT SUM DEFAULT '0' ) AGGREGATE KEY(siteid, citycode, username) DISTRIBUTED BY HASH(siteid) BUCKETS 10;

3. See error
   ERROR 1064 (HY000): errCode = 2, detailMessage = Failed to find enough host in all backends. need: 3
   

Expected behavior
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
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