[Merge] lp:~javier.collado/ubuntu-test-cases/bug1087630 into lp:~ubuntu-server-dev/ubuntu-test-cases/server-tests-raring

[Javier Collado]

Review: Resubmit

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

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https://code.launchpad.net/~javier.collado/ubuntu-test-cases/bug1087630/+merge/142161
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