Support Airflow deployment with clustered Redis

**Description**

Currently, if users try to use Redis in a clustered way (for example using AWS ElastiCache cluster with multiple nodes) they may get the following error:
```
ResponseError: CROSSSLOT Keys in request don't hash to the same slot
```
This error occurs because keys must be in the same hash slot and not just the same node. Similar problems can be also observed when running flower.

**Use case / motivation**

Support Redis in clustered mode or provide information that it is not supported.

**Related Issues**
https://stackoverflow.com/questions/45215164/airflow-crossslot-keys-in-request-dont-hash-to-the-same-slot-error-using-aws-el/48123036
https://gitter.im/apache/incubator-airflow?at=5a4fe070b48e8c3566a54db9
https://aws.amazon.com/premiumsupport/knowledge-center/elasticache-crossslot-keys-error-redis/

The issue names no files, tests, or entry points. Start by examining Airflow and Flower's Redis usage in a clustered AWS ElastiCache setup, then determine whether clustered Redis can be supported or whether documentation should state that it is unsupported. Completion should address the reported CROSSSLOT error.

Written by the indexing model from the issue text.