Update gcr.io/kasten-images/controllermanager Docker tag to v8.0.15

This MR contains the following updates:

Package	Type	Update	Change
gcr.io/kasten-images/controllermanager		patch	8.0.14 -> 8.0.15
gcr.io/kasten-images/controllermanager	ironbank-docker	patch	8.0.14 -> 8.0.15
gcr.io/kasten-images/controllermanager	stage	patch	8.0.14 -> 8.0.15

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