title: Terraform for Azure : storage_os_disk - invalid option
body: <p>tried creating a new Azure VM and have specified options to create a empty storage OS Premium disk as below. Not sure what are the valid options  to create an empty disk of size 100GB.</p>
  
  <pre><code>    resource "azurerm_virtual_machine" "main" {
        name                  = "${var.prefix}-vm"
        location              = "${azurerm_resource_group.main.location}"
        resource_group_name   = "${azurerm_resource_group.main.name}"
        network_interface_ids = ["${azurerm_network_interface.main.id}"]
        vm_size               = "Standard_D4s_v3"
  
        # Uncomment this line to delete the OS disk automatically when 
      deleting the VM
        # delete_os_disk_on_termination = true
  
  
        # Uncomment this line to delete the data disks automatically when 
      deleting the VM
        # delete_data_disks_on_termination = true
  
        storage_image_reference {
          publisher = "Canonical"
          offer     = "CentOS"
          sku       = "7.4"
          version   = "latest"
        }
        storage_os_disk {
          name              = "myosdisk1"
          caching           = "ReadWrite"
          create_option     = "Empty"
          managed_disk_type = "Premium_LRS"
          disk_size_gb      = "100"
        }
        os_profile {
          computer_name  = "hostname"
          admin_username = "testadmin"
          admin_password = "Password1234!"
        }
        os_profile_linux_config {
          disable_password_authentication = false
        }
        tags = {
          environment = "test-nonprod-provisioning"
        }
      }
  </code></pre>