Prime UEC images with apt/git repos

Once we simplify how build_libvirt.sh works with uec images, we lose the niceness of having a multi-stage process that minimize the time/bandwidth for each run of devstack.

The goal was to:

  * pre-install as many of the apts as possible (while not installing services?)
  * pre-clone the git repositories

Perhaps we can use the UEC userdata/metadata functionality to boot a priming step - so we still keep things simple (no mounting loopback, ...) and save a primed image?