Base boxes revamp

This PR consolidates all of the open ones related to building base boxes, I'm currently focusing on some other improvements around it and the ultimate goal is to both lock the base box format for a vagrant-lxc 1.0 release candidate I'm planning to cut at the end of the month and to and move the code related to base box building out into a repository of its own.

_For those interested, I'll provide more information about the changes before cutting the first 1.0 beta_

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