support followers=0 syntax

[lcwangchao]

Users may want to schedule only one leader without any followers. But when we set followers=0, it will be regarded as user did not set it (int has a default value 0).

[xhebox]

/cc @morgo , I've forbidden setting it to 0, which makes sense to me. I want to hear from you.

[morgo]

OK, originally I was the same opinion as @xhebox , but on reconsideration @lcwangchao is correct: consider the case that data can be regenerated. In this case you may not wish to have redundancy.

So we should permit the case of followers=0.