make subscription_id configurable in cli

[mogrogan]

This changes will make the plugin use the subscription_id at the root even when using the cli auth mode

2 packer build were done to test the feature:

packer {
  required_plugins {
    azure = {
      source  = "github.com/hashicorp/azure"
      version = "~> 2"
    }
  }
}

source "azure-arm" "ubuntu" {
  use_azure_cli_auth = true

  # subscription_id = "eeb6d260-e6c0-4ec6-83e0-492c8272798c"

  managed_image_name                = "ubuntu-20-04-{{timestamp}}"
  managed_image_resource_group_name = "packer_test"

  os_type         = "Linux"
  image_publisher = "Canonical"
  image_offer     = "0001-com-ubuntu-server-focal"
  image_sku       = "20_04-lts"

  location = "East US"
  vm_size  = "Standard_DS2_v2"

  azure_tags = {
    Environment = "Dev"
    Created_By  = "Packer"
  }
}

build {
  sources = ["source.azure-arm.ubuntu"]

  provisioner "shell" {
    inline = [
      "echo 'Installing updates...'",
      "sudo apt-get update",
      "sudo apt-get upgrade -y",
      "sudo apt-get install -y nginx",
      "echo 'Installation complete'"
    ]
  }
}

and ...

packer {
  required_plugins {
    azure = {
      source  = "github.com/hashicorp/azure"
      version = "~> 2"
    }
  }
}

source "azure-arm" "ubuntu" {
  use_azure_cli_auth = true

  subscription_id = "eeb6d260-e6c0-4ec6-83e0-492c8272798c"

  managed_image_name                = "ubuntu-20-04-{{timestamp}}"
  managed_image_resource_group_name = "packer_test"

  os_type         = "Linux"
  image_publisher = "Canonical"
  image_offer     = "0001-com-ubuntu-server-focal"
  image_sku       = "20_04-lts"

  location = "East US"
  vm_size  = "Standard_DS2_v2"

  azure_tags = {
    Environment = "Dev"
    Created_By  = "Packer"
  }
}

build {
  sources = ["source.azure-arm.ubuntu"]

  provisioner "shell" {
    inline = [
      "echo 'Installing updates...'",
      "sudo apt-get update",
      "sudo apt-get upgrade -y",
      "sudo apt-get install -y nginx",
      "echo 'Installation complete'"
    ]
  }
}

both build were successful and we have both images in respective subscription

Closes #461

[JenGoldstrich]

Nice work @mogrogan thanks for your first contribution to the Azure plugin!