tests/resource/aws_eks_node_group: Update TestAccAWSEksNodeGroup_Rele…

…aseVersion argument value

Reference: awslabs/amazon-eks-ami#423
Reference: aws/containers-roadmap#771

There is currently no lookup mechanism for these values other than the website. See referenced issue for hopeful path forward via a new SSM Parameter, although there does appear to be a delay between the AMI release and EKS API support.

Previous output from acceptance testing (master and latest version testing):

```
--- FAIL: TestAccAWSEksNodeGroup_ReleaseVersion (1225.88s)
    testing.go:654: Step 0 error: errors during apply:

        Error: error creating EKS Node Group (tf-acc-test-9113909416381506697:tf-acc-test-9113909416381506697): InvalidParameterException: Requested Node Group release version 1.14.7-20190927 is invalid. Allowed release version is 1.14.8-20191213

--- FAIL: TestAccAWSEksNodeGroup_ReleaseVersion (1375.87s)
    testing.go:654: Step 0 error: errors during apply:

        Error: error creating EKS Node Group (tf-acc-test-7174874914828901519:tf-acc-test-7174874914828901519): InvalidParameterException: Requested Node Group release version 1.14.9-20200122 is invalid. Allowed release version is 1.14.8-20191213
```

Output from acceptance testing:

```
--- PASS: TestAccAWSEksNodeGroup_ReleaseVersion (1564.18s)
```

aws/resource_aws_eks_node_group_test.go

@@ -279,10 +279,10 @@ func TestAccAWSEksNodeGroup_ReleaseVersion(t *testing.T) {
 		CheckDestroy: testAccCheckAWSEksNodeGroupDestroy,
 		Steps: []resource.TestStep{
 			{
-				Config: testAccAWSEksNodeGroupConfigReleaseVersion(rName, "1.14.7-20190927"),
+				Config: testAccAWSEksNodeGroupConfigReleaseVersion(rName, "1.14.8-20191213"),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckAWSEksNodeGroupExists(resourceName, &nodeGroup1),
-					resource.TestCheckResourceAttr(resourceName, "release_version", "1.14.7-20190927"),
+					resource.TestCheckResourceAttr(resourceName, "release_version", "1.14.8-20191213"),
 				),
 			},
 			{