Major stable/redis version: remove value inheritance and add optional sentinel support

[javsalgar]

What this PR does / why we need it:

This PR was already reviewed in different local requests. Several major changes are proposed:

• Remove master/slave value inheritance (https://github.com/javsalgar/charts/pull/1): in the long term, the current master-slave value inherintance approach lacked transparency and was difficult to maintain.
• Change slaves from deployments to statefulsets (https://github.com/javsalgar/charts/pull/2): in the current approach (slaves without persistence), in the case of failover, the new slave would have to sync all the data from the very beginning. In terms of performance and I/O consumption, this is problematic. If we have persistence in the slaves, according to the redis documentation (https://redis.io/topics/replication):

When the link between the master and the slave breaks, for network issues or because a timeout is sensed in the master or the slave, the slave reconnects and attempts to proceed with a partial resynchronization: it means that it will try to just obtain the part of the stream of commands it missed during the disconnection.

• Add optional redis-sentinel support (https://github.com/javsalgar/charts/pull/3): adds Redis Sentinel support to the current configuration (requested by several users). This causes that, instead of two redis services (master and slave) exposed, we only have one with two ports
  • 6379: Redis read only access
  • 26379: Redis sentinel

For write operations, the user would need to query sentinel first.

Note: This is disabled by default. The static master-slave approach is the one enabled by default.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes [stable/redis] Making chart deployment HA with Sentinels? bitnami/charts#856

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md

[k8s-ci-robot]

Hi @javsalgar. Thanks for your PR.

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[javsalgar]

@juan131 @tompizmor @prydonius @desaintmartin

[carrodher]

@javsalgar a "How to upgrade to 7.0.0" is welcome in the README.md

[prydonius]

This looks good, thanks! It would be good to add a section to the readme explaining how to enable and use sentinel

[javsalgar]

Thanks @prydonius, in the README file you can find a section regarding enabling sentinel

[juan131]

/ok-to-test

[juan131]

/retest

[cpanato]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, javsalgar

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