azure/resize_disk: Align resource_group computation initialization

This bunch of if...elif...fi is there to reproduce the way the
resource_group and nodename are currently built in the azure infra.

So that we can use those script instead of clicking in the portal ui.

We will either need to:

- factorize that snippet part at some point (which is duplicated in
  add-disk, create-vm and here so far)

- rename our resources in the azure portal