Optional Redis HA deployment for #41

[ihrankouski]

This PR provides an alternative to default "single instance Redis" option:

• Replica set of 3 Redis instances and 3 Sentinels for a failover (Redis Operator must be installed in cluster)
• Since Redigo doesn't support Sentinel, there's a HAProxy based workaround. Inside the pod, there's a shell script that is monitoring master address (asking Sentinels every second). And if address changes then the script refreshes HAProxy configuration without the restart (writing changes to 'stats socket')

[joeholley]

LGTM as we discussed in slack. Thanks very much!