Include certificate validation using CNAME DNS record as part of template

[cloudkj]

Adding this would remove the need for the third manual step in the quick start guide - to manually validate ownership of the domain by creating the CNAME records using the AWS ACM console.

Preliminary research seems to indicate that official CloudFormation support for template-driven certificate validation may be coming, but until then some potential solutions are:

• https://binx.io/blog/2018/10/05/automated-provisioning-of-acm-certificates-using-route53-in-cloudformation/
• feat(aws-certificatemanager): add DNSValidatedCertificate aws/aws-cdk#1797