Fix Redis Setinel configuration

[seanrmurphy]

The current implementation does not have proper Redis Sentinel support, being unable to accept a list of sentinels. This issue also arises in other services(Renku-Python issue here, Renku-ui issue here).

To fix it, it is necessary to:

• provide a mechanism by which a list of sentinels is made available to the gateway
• modify the gateway such that these sentinels are used in the config of the Redis client

The specific way that this info will be provided via the helm chart has not been decided yet.