Debian Stretch released as stable recently, and the init system is
less tightly specified in the base dependencies (for some info, see
[1]). It seems, probably unintentionally, that in the previous
release systemd-sysv was brought in by debootstrap, but that is no
longer happening.
Add systemd as an early dependency of debian-minimal.
Remove the package-installs.yaml as that happens too late (other
things need to know the init system to write out service files, etc
and probe for systemd utils before package-installs). As mentioned, I
do not believe the "only install systemd on testing" idea was actually
working here, because it was being brought in during the initial
debootstrap.
Update some documentation to explain what's going on
[1] https://lists.debian.org/debian-boot/2015/05/msg00156.html
Change-Id: Id67c0cf08728407d234976f9807d3bd71d12f758
