We want to be able to test upgrades from different versions. These
parameters can be used to specify the release we start the upgrade from.
No default value is provided to avoid hard coding a specific release.
The values are automatically set to the latest available release for the
selected API version when left empty (by scripts in metal3-dev-env).
Change-Id: I7eaca37048b9eb3a8a1801e661c537fcec3b7040