Add variable to control the private_dns_zone_id value

[AWSmith0216]

For our internal VPNed subscriptions, the use of the hardcoded System value for private_dns_zone_id does not seem to work. The worker nodes fail to contact the API server with an error similar to:

server can't find adsmit-foo-aks-38c762d3.48995284-7e91-4ff1-bd6f-2ea6fbf3ae54.privatelink.eastus.azmk8s.io: NXDOMAIN

As for why the error occurs, I'm not sure. When set to System, a private DNS zone is created with the appropriate A record that points the private FQDN to the internal IP. The DNS zone link to the vnet is also setup. Why this link doesn't seem to be working as expected, I don't know.

The VNETs in our CIS provided VPNed subscriptions have custom DNS servers set, rather than using the Default (Azure-provided) option. I'm assuming that's related to the problem. According to the terraform doc, setting this value to None means that you're bringing your own DNS servers. When set to None, no private DNS zone is created, there's apparently no private FQDN, and the public FQDN resolves to the private IP from within the VNET.

I suppose why System doesn't work is somewhat irrelevant though. This pull request just makes it something the user can adjust as they wish, rather than having it hardcoded. I also purposely didn't add this variable to the BYO Networking README so it would be considered "use at your own risk".