GCE bring-up: Differentiate NODE_TAGS from NODE_INSTANCE_PREFIX

Kubernetes-side fix to #29074 (GKE-side fix necessary as well)

cluster/gce/gci/configure-helper.sh

@@ -189,8 +189,13 @@ EOF
   fi
   if [[ -n "${NODE_INSTANCE_PREFIX:-}" ]]; then
     use_cloud_config="true"
+    if [[ -n "${NODE_TAGS:-}" ]]; then
+      local -r node_tags="${NODE_TAGS}"
+    else
+      local -r node_tags="${NODE_INSTANCE_PREFIX}"
+    fi
     cat <<EOF >>/etc/gce.conf
-node-tags = ${NODE_INSTANCE_PREFIX}
+node-tags = ${node_tags}
 node-instance-prefix = ${NODE_INSTANCE_PREFIX}
 EOF
   fi