Use separate RG for K8S persistent volumes

[JouHouFin]

Commit 2541caf013b50ba1b2ac7f740b9efbedf13686d3:

• Creates separate resource group for storage needs.
• Creates separate tfstate file for the resource group.
• Switches 'azure-disk-retain' -storage class to use the above resource group
• Creates an Azure rolebinding so that our cluster can access the resource group.

By this commit, persistent volumes created to the separate resource group are not destroyed even if terraform destroy is performed in the root module. However, in the current script the "abandoned" volumes are not reused, so if one wants to collect the information in them, it has to be done manually.

Closes #66

[JouHouFin]

With commit fa1b6d7 using the separate resource group is opt-in. This can be enabled via a variable use_separate_storage_rg set as true.