Redis HA with Sentinel

Along with #47 (closed) & #95 (closed)

Prepare configurable split of the Redis chart to handle HA with Master/Slave and Sentinel.

A point to keep in mind, is the possibility of a portion of the Redis cluster being in a difference K8S cluster. This may be hard to do, according to @marin, because of network visibility expectations in Redis/Sentinel. Keeping K8S federation in mind could help assuage this situation.

Part 1: Start with the assumption that everything will be in the same chart, on the same cluster.

Part 2: Allow for mix and match of in and out of k8s cluster Redis cluster members

cc @twk3