Generalize distributed testing docs

[austin362667]

The Rationales

1. Remove redis args: It used to be that the Ray head used redis to store state and Ray workers connected to the head by referencing the address of the head's Redis server (default port 6379). Ray stopped using redis in 1.11.0 -- to make that transition less disruptive, they made it so that workers still connect to the head by referencing port :6379. source

2. Ensure can run single node cluster on MacOS: Users might want a quick start on their own Mac dev box locally, and be aware that multi-node Ray clusters are only supported on Linux. So, I make it optional. source

3. Hint user to simply connect to the cluster: if user already created the clusters in command line, they can just connect to it via ray.init() without specifying required resources.

[andygrove]

Thanks @austin362667