Add support for Redis Sentinel

[mgruner]

What this PR does / why we need it

• Zammad 7.0 will bring support for Redis Sentinel. This PR is supposed to integrate it with the Helm chart.

Checklist

• Chart Version bumped

[monotek]

Maybe start with bitnamisecure repo directly.
Example: #368

We could also merge both PRs and make it the zammad 7.0 updater PR.
Thoughts?

[mgruner]

TODO revert this change, for testing only

[mgruner]

Maybe start with bitnamisecure repo directly. Example: #368

We could also merge both PRs and make it the zammad 7.0 updater PR. Thoughts?

@monotek I'm not sure what the best way is. We could have one branch with all changes for Zammad 7.0 (#348), where we merge other MRs like this one as squash merges. We'd have to change to the develop tag in this branch, and then finally on merging set it back to a normal 7.0 value. This might lead to a MR with several files changed.

Alternative would be to keep multiple MRs like this one around, and merge them one after the other after Zammad 7.0 is available, resulting in multiple new chart versions.

[mgruner]

@klml here are the changes for Redis Sentinel support. The Zammad image for develop has it already, so if you use this branch with the develop image, you can create a test deployment with Redis Sentinel. Could you do that and provide us with your feedback on it, please?