Set instance_weight = 0 in config doesn't mean no partition assigned to the instance

[i3wangyi]

Problem
The issue is raised by our internal client who sets the instance_weight = 0 in instance/config. Instead of expecting zero partitions assigned to it, based on our Helix controller algorithm, there's still a good chance some partitions assigned to the instance.

Code of issue
After investigation, the issue was due to the current CardDealingAlgorithm. It treats instance weight as a comparator factor or preference, rather than respecting the weight = 0.

https://github.com/apache/helix/blob/d5bf3ad410e0a54b1e79d9570d3e7897c0e2c947/helix-core/src/main/java/org/apache/helix/controller/rebalancer/strategy/crushMapping/CardDealingAdjustmentAlgorithmV2.java

Solution
There's no way to strictly follow all the weight setting of instances now. Because the algorithm is not designed for the purpose. But for the extreme case of instance weight = 0, the current workaround solution is add a simple check to skip the assignment of the instance.