Revert to the original organisation hierarchy after a delete stack

[jeromevdl]

Following the idempotent feature (#13), we need to make sure after a stack deletion, the OU hierarchy revert to its original state:

• Existing accounts moved to a new OU should be moved back to their initial OU (if it still exists) or at the root.
• Newly created OUs should be removed (after having moved the accounts).
• Newly created accounts should be moved to the root or a special OU (Recycle).