cloud-init hook order firing changed in saucy

Starting with saucy, cloud-init change its hook firing order.

vms.VM.set_uploaded_scripts() and vms.VM._hook_script_path() assumes the ubuntu user exists which is not true anymore.

boothook is now fired before the user is created so the uploaded files has to be stored elsewhere (/tmp is guaranteed to exist).

Then, once the command is triggered, the ubuntu user exists and the script can exectute successfully.