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/