install doesn't prompt for installing debathena-thirdparty

[jweiss]

When doing a (PXE, but I don't think it's specific to that) install of anything that's not -cluster, the installer doesn't ask me about whether I want debathena-thirdparty. It always assumes I don't and notes that I'm not getting it. It does ask about extra-software (except on workstation where it is required) and makes the appropriate notes depending on my answer.