update method for preseed management

[Max Brustkern]

It would be cool if we had a method to combine preseeds. I'm thinking something like:
PreseedA.update(PreseedB)
Anything not in PreseedA from PreseedB would be added, and any questions in both would use the answers from PreseedB. This could make the rewriting a lot simpler. I'm imagining something like:

PreseedWeGetPassedIn.update(DefaultUTAHPreseed)
if installtype = desktop:
    PreseedWeGetPassedIn.update(DesktopOnlyPreseed)
etc.