The `subscription_id` should also be set on terraform backend for azure

[mvantellingen]

We need to be able to set the subscription_id on the terraform state backend for azure when it should use a non-default

[demeyerthom]

@mvantellingen the azurem backend has a lot of potential options: https://developer.hashicorp.com/terraform/language/settings/backends/azurerm#configuration-variables

The ones that can be set in mach-composer are only the required ones for Managed Identity (MSI) authentication. However there are other options where resource_group_name is not required.

I am thinking of removing resource_group_name as a required field, and instead adding the other options to the backend to be generated.

That, or still make resource_group_name not required and have the remainder be set through the environment variables, pretty much as we do with AWS and GCP.

What are your thoughts on this?