[RedisDB::Cluster] want to use `statup_nodes` for `_initialize_slots` always

[karupanerura]

For example, in Amazon ElastiCache's Redis Cluster, it is expected to always refer to the Configuration Endpoint.
SEE: https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/Endpoints.html

We have also found that there are cases where the nodes cannot be retrieved correctly if the cluster configuration is changed without referring to the Configuration Endpoint.(Unfortunately, I cannot explain the details of the problem.)

This problem is solved to initialize a slot with reference to the startup_nodes.

I think it should be changeable the _initialize_slots behavior in some way.

[karupanerura]

Pushed my solution idea: #41

[karupanerura]

@trinitum how about it?