use checkedConnectCluster to Redis in Gundeck

[stefanwire]

https://wearezeta.atlassian.net/browse/BE-571

Checklist

• The PR Title explains the impact of the change.
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[jschaul]

This looks like a good change overall. Writing automated integration tests for this might be a bit tricky; so let's merge this to develop, and perform a manual test on staging I'd say. Thanks for this PR!