[cern] Use SNAPSHOT_VALIDATION_WEBHOOK_ENABLED variable instead

This MR fixes wrong variable name from SNAPSHOT_VALIDATION_WEBHOOK to SNAPSHOT_VALIDATION_WEBHOOK_ENABLED. It's defined in https://gitlab.cern.ch/cloud-infrastructure/magnum/-/blob/0485f4455c162007fd2917972a857dd073133773/magnum/drivers/common/templates/kubernetes/fragments/write-heat-params-master.sh#L171 .

magnum/drivers/common/templates/kubernetes/helm/cern-chart.sh

@@ -101,7 +101,7 @@ ${NVIDIA_GPU_VALUES}
     snapshot-controller:
       enabled: ${SNAPSHOT_CONTROLLER_ENABLED}
     snapshot-validation-webhook:
-      enabled: ${SNAPSHOT_VALIDATION_WEBHOOK}
+      enabled: ${SNAPSHOT_VALIDATION_WEBHOOK_ENABLED}
     node-problem-detector:
       enabled: ${NPD_ENABLED}
     base: