title: how to add existing subnetwork for gke cluster using terraform
body: <p>I am adding vpc, subnet and gke cluster referring to that vpc and subnet. However, vpc and subnet are getting created but while creating gke cluster it is throwing error that </p>
  
  <pre><code>Create cluster cannot be created with existing subnetwork.
  </code></pre>
  
  <p>I have added the below script which is creating VPC, subnetwork and gke cluster. Please see the terraform script below</p>
  
  <pre><code>provider "google" {
    credentials = "gkecredential"
    project     = "abc"
    region      = "us-central1"
  }
  
  resource "google_compute_network" "vpc" {
    name                    = "test"
    auto_create_subnetworks = "false"
  }
  
  resource "google_compute_subnetwork" "gkesubnet" {
    name          = "testgkesubnet"
    ip_cidr_range = "10.105.0.0/24"
    network       = "${google_compute_network.vpc.self_link}"
    region        = "us-central1"
  
    secondary_ip_range {
      range_name    = "container-range-1"
      ip_cidr_range = "10.75.0.0/20"
    }
  
    secondary_ip_range {
      range_name    = "service-range-1"
      ip_cidr_range = "10.8.0.0/14"
    }
  }
  
  resource "google_container_cluster" "gcp_kubernetes" {
    name               = "gkecluster"
    zone               = "us-west1-a"
    initial_node_count = "1"
    network            = "${google_compute_network.vpc.self_link}"
    subnetwork         = "${google_compute_network.gkesubnet.self_link}"
  
    additional_zones = [
      "us-west1-b",
      "us-west1-c",
    ]
  
    private_cluster_config {
      enable_private_endpoint = false
      enable_private_nodes    = true
      master_ipv4_cidr_block  = "172.16.0.0/28"
    }
  
    master_auth {
      username = "abc"
      password = "abc"
    }
  
    master_authorized_networks_config {
      cidr_blocks = [
        {
          cidr_block   = "10.102.0.0/24"
          display_name = "Office"
        },
      ]
    }
  
    ip_allocation_policy {
      create_subnetwork = true
    }
  
    node_config {
      oauth_scopes = [
        "https://www.googleapis.com/auth/compute",
        "https://www.googleapis.com/auth/devstorage.read_only",
        "https://www.googleapis.com/auth/logging.write",
        "https://www.googleapis.com/auth/monitoring",
      ]
  
      labels {
        this-is-for = "dev-cluster"
      }
  
      tags = ["dev", "work"]
    }
  }
  </code></pre>