Redis Sentinel support for api-gateway

[retheeshskumar]

Problem description

api-gateway Guadian service doesn't support Redis sentinel. Redis Sentinel provides high availability for Redis when not using Redis Cluster.

Step to reproduce

Steps to reproduce the behavior:

1. Build docker image
2. Run in Kubernetes environment.
3. Currently it has option to provide only Redis hosts and not Sentinel hosts list.

Expected behavior

The api-gateway shouldwork properly with Redis or Redis sentinel

[prernaadev01]

@retheeshskumar thank you so much for your PR. Can you please also help us in documenting these changes if possible.