Allow using a different subscription ID for the VNet

[sarandia]

What would you like to be added:

We'd like to use a vnet from a different subscription.
If the VNET is in a different sub but CCM doesn't know, then CCM will fail to reconcile LB frontend IPs and NSG rules when there's a k8s internal LB service. CCM needs to read vNet information when reconciling those resources. Today, there is a field in azure.json called "networkSubscriptionID" but that's applied to all networking resources, including LBs, NSGs, vNET

Would it be possible to use a different subscription ID only for the VNet (subnets)?

Why is this needed:

[feiskyer]

That's right. We have https://cloud-provider-azure.sigs.k8s.io/install/configs/#host-network-resources-in-different-aad-tenant-and-subscription today, which is not specific for VNet. So this would be a new feature request.

@sarandia would you like to contribute this feature?

[sarandia]

@feiskyer thank you; do you think you can give me some pointers on where this needs to be changed? I assume we'll need to change whereever in the code is trying to use a subnet client?