use official debian image in makebuildserver
Debian has official images on atlas (vagrants image repository) It would make sense to use those instead of hosting the base image for the buildserver by ourselves. (which is currently down, also see: #356 (closed))
This is also an opportunity to upgrade the buildserver image from jessie to stretch.
I think it would be okay defaulting to "debian/stretch" but also add an option eg. makebuildserver --base-box BOXNAME
to support creating a buildservers from custom images/boxes.