fix(elbv2): use correct prop for TargetGroup name

TargetGroup construct passes targetGroupName prop to underlying
CfnTargetGroup, but the correct name is Name, not TargetGroupName.

fixes aws#1674

packages/@aws-cdk/aws-elasticloadbalancingv2/lib/shared/base-target-group.ts

@@ -213,7 +213,7 @@ export abstract class TargetGroupBase extends cdk.Construct implements ITargetGr
     this.targetType = baseProps.targetType;
 
     this.resource = new CfnTargetGroup(this, 'Resource', {
-      targetGroupName: baseProps.targetGroupName,
+      name: baseProps.targetGroupName,
       targetGroupAttributes: new cdk.Token(() => renderAttributes(this.attributes)),
       targetType: new cdk.Token(() => this.targetType),
       targets: new cdk.Token(() => this.targetsJson),

packages/@aws-cdk/aws-elasticloadbalancingv2/test/alb/test.listener.ts

@@ -114,6 +114,26 @@ export = {
     test.done();
   },
 
+  'Can configure name on TargetGroups'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.VpcNetwork(stack, 'Stack');
+
+    // WHEN
+    new elbv2.ApplicationTargetGroup(stack, 'TargetGroup', {
+      vpc,
+      port: 80,
+      targetGroupName: 'foo'
+    });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ElasticLoadBalancingV2::TargetGroup', {
+      Name: 'foo'
+    }));
+
+    test.done();
+  },
+
   'Can add target groups with and without conditions'(test: Test) {
     // GIVEN
     const stack = new cdk.Stack();