separate statefulset name and statefulset service name in downscaling

[fitz7]

I found that the rollout-operator was not calling the right pod endpoint with our mimir installation but was calling mimir-ingester-zone-c-1.mimir-ingester-zone-c.mimir.svc.cluster.local when it should be using the headless service at mimir-ingester-zone-c-1.mimir-ingester-headless.mimir.svc.cluster.local

This led me to issue #125 so checked on our statefulsets and found the serviceName was correctly set to serviceName: mimir-ingester-headless

so I dug around a bit and found that the operator was using the an arg named serviceName in createPrepareDownscaleEndpoints but passing it the statefulset name.

So I've split them to use both the statefulsets name and it's serviceName so that hopefully these requests go to the correct pods

Let me know if this needs changing or if there is another approach that I've missed?

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