Code review comment for lp:~javier.collado/ubuntu-test-cases/bug1087630

Sorry I'm not familiar with the templates in this branch and I didn't know that
there's a template for minimal-virtual that should be used to generate
minimal-virtual.preseed even if both are under version control. Thanks Siva for
the explanation through irc.

I've added a couple of revisions to deal with that:
- make late_command customizable in the templates
- use that in the minimal-virtual template to gather the root partition usage