Initial VM test builds on TravisCI

We're starting a Docker container with Debian/stretch, installing
the Debian package build result (generated via travis.debian.net)
of grml-debootstrap inside it. Then execute grml-debootstrap,
installing to a VM file. Finally boot the generated VM file via
qemu.

We use stdio of qemu's serial console to check for the boot prompt
of the installed Debian system, using goss for running the actual
checks.