Implement better tests for openstack image generation

[Doeme]

See the ideas and things started in branch wip/openstack_image_testing

What we could aim for is starting the openstack image in a qemu emulator, checking on serial out whether the startup looks reasonable, and then test to log-in via ssh.

Alternatively, we could find out why the images closes our serial port and does not start agetty on it, because if it did we could actually log in via stdin/out of the qemu process, and see whether things match there.