[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 .