IDEA: Specify the subscription to be used for each landingzone

[rdbartram]

Since most tenants have more than 1 subscription, in order to to simplify the deployments it would be great if there was a way to specify the target subscription on the stack object. A concrete use case for this is vnet peering over multiple subscription. As of today this requires n+2 rover invocations where n is the number of subscriptions.