extend primary partition to use template.json

If we use the `<Extend>true</Extend>` tag in the `Autounattend.xml` files, then there only is one place to change disk size of the basebox: the template JSON files.

I once had the trouble to increase packer's `disk_size` in the template JSON file, but found out that the VM still only got 60GByte.

This time targeting develop branch. (This is my most wanted feature at GitHub that a maintainer could preselect the base branch for PR's independently to the [default branch](https://help.github.com/articles/setting-the-default-branch/) to be displayed on the Web.)