Deployment of private cluster fails on Azure Stack Hub

Version:

$ openshift-install version
openshift-install 4.10.0-rc.2
built from commit 1ddc64b523042f450f21cc45f1150d29cb01ecc1
release image quay.io/openshift-release-dev/ocp-release@sha256:da16f451dddad3ca26ecd7c5c16423fb3658d7883dadc108977675edf80c696a
release architecture amd64

Platform: Azure Stack Hub
IPI

During the private cluster installation and with using an existing vnet, Terraform fails when the `publish: Internal` is set into the install-config.yaml.

It looks like that there is no default value for the following variables https://github.com/openshift/installer/blob/release-4.10/data/data/azurestack/bootstrap/variables.tf#L1-L9
While they are set for Azure (https://github.com/openshift/installer/blob/release-4.10/data/data/azure/bootstrap/variables.tf#L1-L11)

Here are some type issues (`[]` instead of `null`):

• https://github.com/openshift/installer/blob/release-4.10/data/data/azurestack/bootstrap/main.tf#L102
• https://github.com/openshift/installer/blob/release-4.10/data/data/azurestack/cluster/master/master.tf#L21