Add redis cluster size limit

[jian.song]

In the cluster deployment, we will have a clear number of nodes constraints, such as mongodb inside a shard need to include a three nodes composed of replication set. This will be very clear. But redis did not have such a complete constraint, I collected some information, confirmed the official recommended redis cluster number between 3 [1]to 1000 [2] nodes.

[1]:https://redis.io/topics/cluster-tutorial

Note that the minimal cluster that works as expected requires to contain at least three master nodes. For your first tests it is strongly suggested to start a six nodes cluster with three masters and three slaves.

[2]:https://redis.io/topics/cluster-spec

The key space is split into 16384 slots, effectively setting an upper limit for the cluster size of 16384 master nodes (however the suggested max size of nodes is in the order of ~ 1000 nodes).