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Retry node pool deletion in autoscaling tests.
**What this PR does / why we need it**:
Currently, when there is already an operation running on the cluster, the node pool deletion will fail. This will in turn make other tests that try to add extra node pool fail.
With this PR we will retry the node pool deletion operation 3 times over the course of ~15 minutes which will hopefully be enough to finish any operations running on the cluster.
This will deflake autoscaling e2e tests on gke regional clusters.
**Release note**:
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NONE
```
@aleksandra-malinowska
cd570c092b