Attempt to delete node-template before retry

I don't think we've seen this commonly, so I'm proposing a simplistic
fix for now.

Fixes #20878

cluster/gce/util.sh

@@ -396,6 +396,11 @@ function create-node-template {
         echo -e "${color_yellow}Attempt ${attempt} failed to create instance template $template_name. Retrying.${color_norm}" >&2
         attempt=$(($attempt+1))
         sleep $(($attempt * 5))
+
+        # In case the previous attempt failed with something like a
+        # Backend Error and left the entry laying around, delete it
+        # before we try again.
+        gcloud compute instance-templates delete "$template_name" --project "${PROJECT}" &>/dev/null || true
     else
         break
     fi