Proposed title of this feature request?

Currently there is no way to define PrivateDNS zone in install-config during installation.

2. What is the nature and description of the request?  — New request
3. Why does the customer need this? (List the business requirements here)  — Customer is using their internal DNS for installation and the installation is failing everytime. They need to link the privatedns zone manually with the vnet to get installation complete.  Customer don't want to manually link everytime they doing an installation, hence they want to specify their existing private dns zone in install-config which is already linked with the vnet. 

4. List any affected packages or components. — installer