title: Cannot retrieve value from context provider ami since account/region are not specified at the stack level
body: <p>Hello I have problems when I try to create a stack on AWS CDK</p>
  
  <pre class="lang-js prettyprint-override"><code>    import * as cdk from '@aws-cdk/core';
      import ec2 = require('@aws-cdk/aws-ec2');
      import { SubnetType } from '@aws-cdk/aws-ec2';
  
      export class FirstDemoStack extends cdk.Stack {
        constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
          super(scope, id, props);
  
          const natGatewayProvider = ec2.NatProvider.instance({
            instanceType: new ec2.InstanceType('t3.micro')
          });
  
          const vpc = new ec2.Vpc(this, 'VPC_Test', {
            cidr: '10.0.0.0/26',
            maxAzs: 1,
            subnetConfiguration:[
              {
                subnetType: ec2.SubnetType.PUBLIC,
                name: 'Ingress',
                cidrMask: 28
              },
              {
                cidrMask: 28,
                name: 'Application',
                subnetType: ec2.SubnetType.PRIVATE
              }
            ],
            natGatewayProvider,
            natGateways:2,
          });
        }
      }
  </code></pre>
  
  <p>So I get the next error</p>
  
  <pre class="lang-sh prettyprint-override"><code>    Cannot retrieve value from context provider ami since account/region are not specified at the stack level. Either configure "env" with explicit account and region when you define your stack, or use the environment variables "CDK_DEFAULT_ACCOUNT" and "CDK_DEFAULT_REGION" to inherit environment information from the CLI (not recommended for production stacks)
  </code></pre>
  
  <p>I know that I need to add the region and account to my stack, but I don't know where</p>