Add EKS cluster auth token data resource

This allows Terraform to authenticate with an EKS cluster via the
Kubernetes provider:

```hcl
resource "aws_eks_cluster" "foo" {
  name = "foo"
}

data "aws_eks_cluster_auth" "foo_auth" {
  name = "foo"
}

provider "kubernetes" {
  host = "${aws_eks_cluster.foo.endpoint}"
  cluster_ca_certificate = "${base64decode(aws_eks_cluster.foo.certificate_authority.0.data)}"
  token = "${data.aws_eks_cluster_auth.foo_auth.token}"
}
```

The auth logic was extracted from
https://github.com/heptio/aws-iam-authenticator because of lack of
documentation from AWS. Basically, the token is a signed URL for the
GetCallerIdentity action with a custom header. The URL is then base64
encoded and prefixed with vendor string.

aws/data_source_aws_eks_cluster_auth.go

@@ -0,0 +1,67 @@
+package aws
+
+import (
+	"encoding/base64"
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/aws/aws-sdk-go/service/sts"
+	"github.com/hashicorp/terraform/helper/schema"
+	"github.com/hashicorp/terraform/helper/validation"
+)
+
+const (
+	clusterIDHeader = "x-k8s-aws-id"
+	v1Prefix        = "k8s-aws-v1."
+)
+
+func dataSourceAwsEksClusterAuth() *schema.Resource {
+	return &schema.Resource{
+		Read: dataSourceAwsEksClusterAuthRead,
+
+		Schema: map[string]*schema.Schema{
+			"name": {
+				Type:         schema.TypeString,
+				Required:     true,
+				ForceNew:     true,
+				ValidateFunc: validation.NoZeroValues,
+			},
+
+			"duration": {
+				Type:     schema.TypeInt,
+				Optional: true,
+				Default:  60,
+			},
+
+			"token": {
+				Type:      schema.TypeString,
+				Computed:  true,
+				Sensitive: true,
+			},
+		},
+	}
+}
+
+func dataSourceAwsEksClusterAuthRead(d *schema.ResourceData, meta interface{}) error {
+	conn := meta.(*AWSClient).stsconn
+	name := d.Get("name").(string)
+	duration := d.Get("duration").(int)
+
+	request, _ := conn.GetCallerIdentityRequest(&sts.GetCallerIdentityInput{})
+	request.HTTPRequest.Header.Add(clusterIDHeader, name)
+
+	url, err := request.Presign(time.Duration(duration) * time.Second)
+	if err != nil {
+		return fmt.Errorf("error presigning request: %v", err)
+	}
+
+	log.Printf("[DEBUG] Generated request: %s", url)
+
+	token := v1Prefix + base64.RawURLEncoding.EncodeToString([]byte(url))
+
+	d.SetId(time.Now().UTC().String())
+	d.Set("token", token)
+
+	return nil
+}

aws/data_source_aws_eks_cluster_auth_test.go

@@ -0,0 +1,56 @@
+package aws
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/hashicorp/terraform/helper/resource"
+	"github.com/hashicorp/terraform/terraform"
+)
+
+func TestAccAWSEksClusterAuthDataSource_basic(t *testing.T) {
+	dataSourceResourceName := "data.aws_eks_cluster_auth.test"
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:  func() { testAccPreCheck(t) },
+		Providers: testAccProviders,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccCheckAwsEksClusterAuthConfig_basic,
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAwsEksClusterAuthToken(dataSourceResourceName),
+				),
+			},
+		},
+	})
+}
+
+func testAccCheckAwsEksClusterAuthToken(n string) resource.TestCheckFunc {
+	return func(s *terraform.State) error {
+		rs, ok := s.RootModule().Resources[n]
+		if !ok {
+			return fmt.Errorf("Can't find EKS Cluster Auth resource: %s", n)
+		}
+
+		if rs.Primary.ID == "" {
+			return fmt.Errorf("EKS Cluster Auth resource ID not set.")
+		}
+
+		name := rs.Primary.Attributes["name"]
+		if expected := "foobar"; name != expected {
+			return fmt.Errorf("Incorrect EKS cluster name: expected %q, got %q", expected, name)
+		}
+
+		if rs.Primary.Attributes["token"] == "" {
+			return fmt.Errorf("Token expected to not be nil")
+		}
+
+		return nil
+	}
+}
+
+const testAccCheckAwsEksClusterAuthConfig_basic = `
+data "aws_eks_cluster_auth" "test" {
+	name = "foobar"
+}
+`

aws/provider.go

@@ -206,6 +206,7 @@ func Provider() terraform.ResourceProvider {
 			"aws_efs_mount_target":                   dataSourceAwsEfsMountTarget(),
 			"aws_eip":                                dataSourceAwsEip(),
 			"aws_eks_cluster":                        dataSourceAwsEksCluster(),
+			"aws_eks_cluster_auth":                   dataSourceAwsEksClusterAuth(),
 			"aws_elastic_beanstalk_hosted_zone":      dataSourceAwsElasticBeanstalkHostedZone(),
 			"aws_elastic_beanstalk_solution_stack":   dataSourceAwsElasticBeanstalkSolutionStack(),
 			"aws_elasticache_cluster":                dataSourceAwsElastiCacheCluster(),