Fix failure to set imagePullSecrets for user-placeholder pods (scheduling.userPlaceholder.image config added)

[michaellzc]

Updated summary by Erik

prePuller.pause.image.name|tag was used to set the user-placeholder pods image, but the user-placeholder pods didn't make use of the prePuller.pause.image.pullPolicy|pullSecrets or any other config to set imagePullPolicy or imagePullSecrets. With this PR the dedicated option scheduling.userPlaceholer.image and respective fields are added and respected for the user-placeholder pods.

Original PR summary

prePuller.pause.image.pullSecrets was already defined in values.yaml but it was never actually used in the placeholder statefulset template.

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[consideRatio]

It was quite weird that the prePuller.pause.image was used in userPlaceholder instead of scheduling.userPlaceholder.image, so I adjusted this PR that has a very valid point to instead of adding to the weird behavior - creating a dedicated configuration for the userPlaceholder image.

[consideRatio]

Thanks @michaellzc! I wanted to resolve this in a slightly different way, does it make sense to you? I pushed a commit and update the PR description.

[michaellzc]

Thanks @michaellzc! I wanted to resolve this in a slightly different way, does it make sense to you? I pushed a commit and update the PR description.

lgtm 👍

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