azure-arm builder does not allow subscription_id selection when using managed identity authentication

_This issue was originally opened by @dougbw as hashicorp/packer#9140. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

#### Overview of the Issue

Currently it is impossible to specify a subscription_id when using managed identity authentication, as when **only** subscription_id is provided packer assumes an interactive login is desired.

In our case we need the base images to be stored in a different subscription to on running our CI/CD pipeline build agents. A single managed identity can have access to many Azure subscriptions.

#### Reproduction Steps

- Create a user assigned managed identity
- Grant the identity Azure RBAC permissions to multiple subscriptions
- Assign the managed identity to a vm
- On the vm execute `az login --identity` and you should see output for multiple subscriptions
- Provide a subscription_id to the azure-arm builder in packer and you will be stuck on an interactive login