Add cleanup in post demo section (#226)

Author: tungbq

demo/provistion-azure-vm/README.md

@@ -57,13 +57,18 @@ Enter 'yes' to confirm
 - Now you can go to Azure Portal to verify your VM: https://portal.azure.com/#home
 - Or ping the Public IP shown in the console
 
-## Destroy Terraform
-
+## Cleanup
+Destroy the resources with terraform if they are not used anymore:
 ```bash
 # In devops-toolkit contaienr env
 terraform destroy
 ```
 
+And also remove the demo docker container with this command:
+```bash
+devops-toolkit-cli cleanup demo_azure_tf
+```
+
 ## Conclusion
 
 You can see that in just a few steps, you can use Terraform to provision the Azure VM without install terraform on the host machine!