[RFE] - OpenShift 4 IPI + AWS: use CNAME instead of A Alias records

Issue

• As an enterprise customer, OpenShift 4 should CNAME records instead of A Alias records in AWS Route53, because CNAMEs conform to the RFC DNS standard whereas A Alias records do not.
• For enterprise companies, only RFC conforming Route53 records are synced into the enterprise DNS server. Therefore, with the current state of the openshift-installer the "api." URL of an OpenShift 4 cluster is only reachable from within the AWS VPC but not from all the company computers.
• As a proof-of-concept for this approach, there is a workaround to change those records https://github.com/openshift/installer/blob/release-4.3/data/data/aws/route53/base.tf#L50 in the installer to CNAMEs and built a customized version of the installer. With this change, it is possible to resolve the API from the company computers.
• Hence, It would be great to introduce this change into the installer upstream.