Update the Debian build infrastructure

[laeti-tia]

There are a few things that we need to upgrade in our current Debian build infrastructure. This epic is to track all that. The goals are:

• put Debian 7 and Debian 8 builds on 2 different build hosts, to help speed up builds
• be clearer in the use of pbuilder chroots so that dependencies for snapshot and release builds are better handled (that means having a different chroot for snapshot and release, per architecture)
• run more tests during the build (unit tests) and after the build (integration and packaging tests)
• create scripts and documentation to be able to deploy a new Debian building host anywhere when needed (quick provision of new building host)