fix(ecs): cannot disable container insights of an ECS cluster (#9151)

fixes #9149 

The clusterSettings should be explicitly defined even when container insights is disabled, because CFn doesn't allow to remove the existing settings (as described in the issue.)

By this change, users who sets `containerInsights` prop `false` will see a diff like below, but it won't be a problem because by default container insights is disabled hence no actual change to existing resources: 

```
Resources
[~] AWS::ECS::Cluster cluster cluster611F8AFF 
 └─ [+] ClusterSettings
     └─ [{"Name":"containerInsights","Value":"disabled"}]
```

To prevent the existing tests from failing, the clusterSettings is only defined when `containerInsight` prop is explicitly defined(i.e. `true` or `false`, not `undefined`.)

----

*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-ecs/lib/cluster.ts

@@ -124,8 +124,10 @@ export class Cluster extends Resource implements ICluster {
       physicalName: props.clusterName,
     });
 
-    const containerInsights = props.containerInsights !== undefined ? props.containerInsights : false;
-    const clusterSettings = containerInsights ? [{ name: 'containerInsights', value: 'enabled' }] : undefined;
+    let clusterSettings = undefined;
+    if (props.containerInsights !== undefined) {
+      clusterSettings = [{ name: 'containerInsights', value: props.containerInsights ? 'enabled' : 'disabled' }];
+    }
 
     const cluster = new CfnCluster(this, 'Resource', {
       clusterName: this.physicalName,

packages/@aws-cdk/aws-ecs/test/test.ecs-cluster.ts

@@ -1457,7 +1457,27 @@ export = {
     test.done();
   },
 
-  'default container insights undefined'(test: Test) {
+  'disable container insights'(test: Test) {
+    // GIVEN
+    const app = new cdk.App();
+    const stack = new cdk.Stack(app, 'test');
+
+    new ecs.Cluster(stack, 'EcsCluster', { containerInsights: false });
+
+    // THEN
+    expect(stack).to(haveResource('AWS::ECS::Cluster', {
+      ClusterSettings: [
+        {
+          Name: 'containerInsights',
+          Value: 'disabled',
+        },
+      ],
+    }, ResourcePart.Properties));
+
+    test.done();
+  },
+
+  'default container insights is disabled'(test: Test) {
     // GIVEN
     const app = new cdk.App();
     const stack = new cdk.Stack(app, 'test');