chore(eks): add KubernetesVersion 1.17 support (aws#9048)

chore(eks): KubernetesVersion 1.17 support

- Add a new KubernetesVersion enum option for `1.17`
See https://aws.amazon.com/tw/about-aws/whats-new/2020/07/amazon-eks-supports-kubernetes-version-1-17/

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-eks/lib/cluster.ts

@@ -292,6 +292,11 @@ export class KubernetesVersion {
    */
   public static readonly V1_16 = KubernetesVersion.of('1.16');
 
+  /**
+   * Kubernetes version 1.17
+   */
+  public static readonly V1_17 = KubernetesVersion.of('1.17');
+
   /**
    * Custom cluster version
    * @param version custom version number