Fix image_commands and user_packages

Hi,
here are a few patches regarding image_commands and user_packages.

image_commands: various fixes to match updates on the *base* (volume, filesystem,...). It was not working anymore.

user_packages: fix the issue #119, installing the packages at system modification instead of debootstrap due to described issue.
I also changed apt.py to use the debootstrap manifest mirror (or the default one). Indeed, it was set to a default debian mirror site, and may differ from bootstrap selected mirror. For user_packages download, it leaded to long downloads in my case versus my selected mirror (and different apt keys), or for final user it may not match the expected default mirror from the image creator.

Regarding user_packages I think you are rewriting it in python-WIP so you may need to adapt modifications in your branch.