Redis Enterprise Support

We need to make possible for customers to use 3scale on-prem with Redis Enterprise.
One known implication of this is that it needs to be possible to configure components to establish connections to single instance/single db Redis.

Note (from email with dortiz-1):
Redis cluster is a way provided by Redis to shard data across multiple Redis instances. This was introduced in Redis 4.0. In order to use this, you need a client that supports Redis cluster. In the case of the 3scale backend, we use an old version of the client that does not support redis cluster. I don't know whether the system component supports this or not.

I think confusion is caused because Redis enterprise also offers a way to shard data across multiple Redis instances. However, it does not require a client that supports Redis cluster. The reason is that instead of configuring all the instances in "cluster mode", Redis Enterprise uses an intermediate proxy that's responsible for the sharding.