Consider deprecating the CertificateARN Cfn template parameter

[phitoduck]

Hi there!

Wouldn't it be a nice user experience if you didn't have to manually register an ACM cert for the Metaflow UI? I believe that's possible now.

I believe that AWS CloudFormation directly supports automatically validating ACM certificates. Whereas currently the CloudFormation template requires the user to validate a certificate for the UI themselves and then plug in the ARN, we could deprecate that value and take care of it for them.

The option for automatic validation is documented here:

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-certificatemanager-certificate-domainvalidationoption.html

Here is the parameter in the existing template:

https://github.com/outerbounds/metaflow-tools/blob/master/aws/cloudformation/metaflow-cfn-template.yml#L84-L87