Retrieve current OAuth access token from google_client_config data so…

…urce (hashicorp#1277)

* Added access_token field to google_client_config data source

* Refined documentation of google_client_config

docs/d/datasource_client_config.html.markdown

@@ -20,6 +20,25 @@ output "project" {
 }
 ```
 
+## Example Usage: Configure Kubernetes provider with OAuth2 access token
+
+```tf
+data "google_client_config" "default" {}
+
+data "google_container_cluster" "my_cluster" {
+  name   = "my-cluster"
+  zone   = "us-east1-a"
+}
+
+provider "kubernetes" {
+  load_config_file = false
+
+  host = "https://${google_container_cluster.my_cluster.endpoint}"
+  token = "${data.google_client_config.default.access_token}"
+  cluster_ca_certificate = "${base64decode(google_container_cluster.my_cluster.master_auth.0.cluster_ca_certificate)}"
+}
+```
+
 ## Argument Reference
 
 There are no arguments available for this data source.
@@ -31,3 +50,5 @@ In addition to the arguments listed above, the following attributes are exported
 * `project` - The ID of the project to apply any resources to.
 
 * `region` - The region to operate under.
+
+* `access_token` - The OAuth2 access token used by the client to authenticate against the Google Cloud API.