Configure Route53 to CNAME to public DNS hostname

Instead of using an A record to the public IP of the instance, this pull
request adds the option to configure the Route53 to point to the public
DNS hostname (CNAME) instead.
Fix the CNAME record to point to the dns_hostname

Also use the configured TTL. I think missing this out causes Route53 to
get confused when you remove and add record sets
Clearer option description
Merge pull request #219 from HealthUnlocked/route53-use-public-dns-name

Configure Route53 to CNAME to public DNS hostname