ecs-patterns/ApplicationLoadBalancedFargateService fails to deploy in VPC with multiple subnets in same AZ

The ApplicationLoadBalancedFargateService construct (and possibly other related constructs) fails to deploy when there exist multiple public subnets in the same AZ. It appears that the construct selects _all_ public subnets from the provided VPC by default.

### Reproduction Steps

I was following the "ECS Example" described at https://docs.aws.amazon.com/cdk/latest/guide/ecs_example.html but utilizing an existing VPC (with 7 pre-existing public subnets) instead of creating a new VPC like the example prescribes.

### Error Log

```
7/16 | 10:00:14 AM | CREATE_FAILED | AWS::ElasticLoadBalancingV2::LoadBalancer | DevStack/MyFargateService/LB (MyFargateServiceLBDE830E97) A load balancer cannot be attached to multiple subnets in the same Availability Zone (Service: AmazonElasticLoadBalancingV2; Status Code: 400; Error Code: InvalidConfigurationRequest; Request ID: 4915531e-c85f-40f6-93ff-0a899d948d5e)
new BaseLoadBalancer (/mnt/c/git-repos/atlassian-backup/node_modules/@aws-cdk/aws-elasticloadbalancingv2/lib/shared/base-load-balancer.ts:138:22)
\_ new ApplicationLoadBalancer (/mnt/c/git-repos/atlassian-backup/node_modules/@aws-cdk/aws-elasticloadbalancingv2/lib/alb/application-load-balancer.ts:64:5)
\_ new ApplicationLoadBalancedServiceBase (/mnt/c/git-repos/atlassian-backup/node_modules/@aws-cdk/aws-ecs-patterns/lib/base/application-load-balanced-service-base.ts:300:81)
\_ new ApplicationLoadBalancedFargateService (/mnt/c/git-repos/atlassian-backup/node_modules/@aws-cdk/aws-ecs-patterns/lib/fargate/application-load-balanced-fargate-service.ts:92:5)
\_ new AtlassianBackupStack (/mnt/c/git-repos/atlassian-backup/lib/atlassian-backup-stack.ts:24:5)
\_ Object. (/mnt/c/git-repos/atlassian-backup/bin/atlassian-backup.ts:14:15)
\_ Module._compile (internal/modules/cjs/loader.js:778:30)
\_ Module.m._compile (/mnt/c/git-repos/atlassian-backup/node_modules/ts-node/src/index.ts:814:23)
\_ Module._extensions..js (internal/modules/cjs/loader.js:789:10)
\_ Object.require.extensions.(anonymous function) [as .ts] (/mnt/c/git-repos/atlassian-backup/node_modules/ts-node/src/index.ts:817:12)
\_ Module.load (internal/modules/cjs/loader.js:653:32)
\_ tryModuleLoad (internal/modules/cjs/loader.js:593:12)
\_ Function.Module._load (internal/modules/cjs/loader.js:585:3)
\_ Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
\_ main (/mnt/c/git-repos/atlassian-backup/node_modules/ts-node/src/bin.ts:226:14)
\_ Object. (/mnt/c/git-repos/atlassian-backup/node_modules/ts-node/src/bin.ts:485:3)
\_ Module._compile (internal/modules/cjs/loader.js:778:30)
\_ Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
\_ Module.load (internal/modules/cjs/loader.js:653:32)
\_ tryModuleLoad (internal/modules/cjs/loader.js:593:12)
\_ Function.Module._load (internal/modules/cjs/loader.js:585:3)
\_ Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
\_ findNodeScript.then.existing (/home/fshields/.nvm/versions/node/v10.18.1/lib/node_modules/npm/node_modules/libnpx/index.js:268:14)
```

### Environment

- **CDK Version : 1.21.1 (build 842cc5f)**
- **Framework Version:**
- **OS : Debian GNU/Linux 9 (stretch)**
- **Language : TypeScript 3.7.5**

### Other

---

This is :bug: Bug Report

Start by reading lib/base/application-load-balanced-service-base.ts and lib/fargate/application-load-balanced-fargate-service.ts at the stack-trace locations, then reproduce the ECS example with an existing VPC containing multiple public subnets in one Availability Zone. Done means the construct can deploy successfully without attaching multiple same-AZ subnets to the load balancer.

Written by the indexing model from the issue text.