Switch Omnibus Container Registry to be domain based

Follow up from #212 (closed)

On reflection we should move Omnibus Registry to use a subdomain by default for the following reasons:

• Same domain requires opening up a port in firewalls that's a security sniff (and requires both Terraform and Ansible maintenance)
• It would match Helm charts design (Cloud Native Hybrid)
• It's arguably the right solution for larger multi-node environments

While there are defaults here in Omnibus they're arguably not for multi-node setups and the range of options supported leave it up to the user. Using a domain approach will allow users to change the URL as desired as well.

This will be a disruptive change for users who are using Registry on Omnibus environments with GET 2.5.0 in certain circumstances but there will be path to migrate over.