Change modificated file in rollback test

- We were trying to modify upgraders code from venv
but venv is created only after running upgrade. So now
we are modifying /upgrade/releases/metadata.yaml file.
Fix is temporary for 6.1 and will be changed to use fake
upgrader

Change-Id: Ie94bf6498f7ccc48dc17e539b72eb131e90774f4
Closes-Bug: #1402630