Comment 9 for bug 1533587

You wouldn't have to install it yourself, it should be picked automatically.

Also, when you get to the partitioning dialogs, you can hit Go Back to get to the main menu, select "Execute a shell", and execute the following command to check that partman-prep is installed:

grep 'Package: partman-prep' /var/lib/dpkg/status

Still, it should be shipped together with the base image in your case.