fix(eks): remove 'const' from NodeType enum

'const enums' are inlined at the usage site by TypeScript and so the
generated type will not be in the JavaScript source code in the
assembly, even though the declaration will be there.

This leads to "symbol not found" errors upon trying to load it.

https://www.typescriptlang.org/docs/handbook/enums.html#const-enums

Fixes #1969

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

@@ -37,7 +37,7 @@ const LATEST_KUBERNETES_VERSION = '1.11';
 /**
  * Whether the worker nodes should support GPU or just normal instances
  */
-export const enum NodeType {
+export enum NodeType {
   /**
    * Normal instances
    */
@@ -121,4 +121,4 @@ function parseTable(contents: string): {[type: string]: {[region: string]: strin
     [NodeType.GPU]: gpuTable,
     [NodeType.Normal]: normalTable
   };
-}
+}