Install systemd earlier for Stretch

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

diskimage_builder/elements/debian-minimal/README.rst

@@ -11,4 +11,6 @@ setting can be found in the element's ``environment.d`` directory in
 the variable ``DIB_RELEASE``.  If a different release of Debian should
 be created, the variable ``DIB_RELEASE`` can be set appropriately.
 
+Note that this element installs ``systemd-sysv`` as the init system
+
 .. element_deps::

diskimage_builder/elements/debian-minimal/package-installs.yaml

@@ -2,5 +2,3 @@ linux-image-amd64:
   arch: amd64
 linux-image-686:
   arch: i386
-systemd:
-systemd-sysv:

diskimage_builder/elements/debian-minimal/pkg-map

@@ -1,14 +0,0 @@
-{
-  "release": {
-    "debian": {
-      "testing": {
-        "systemd": "systemd",
-        "systemd-sysv": "systemd-sysv"
-       }
-     }
-   },
-  "default": {
-    "systemd": "",
-    "systemd-sysv": ""
-  }
-}

diskimage_builder/elements/debian-minimal/root.d/75-debian-minimal-baseinstall

@@ -16,6 +16,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# dib-lint: disable=safe_sudo
+
 if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
     set -x
 fi
@@ -54,13 +56,23 @@ $apt_get update
 $apt_get clean
 $apt_get dist-upgrade -y
 
-$apt_get install -y busybox sudo
+to_install=""
+
+# pre-stretch (9.0) brought this in via debootstrap, but init
+# dependencies have narrowed in the container world, so now we add it
+# explicitly here so it's brought in early.
+to_install+="systemd-sysv "
+
+# default required
+to_install+="busybox sudo "
 
 if [ "$DIB_PYTHON_VERSION" == "2" ]; then
-    $apt_get install -y python
+    to_install+="python "
 elif [ "$DIB_PYTHON_VERSION" == "3" ]; then
-    $apt_get install -y python3
+    to_install+="python3 "
 else
     echo "ERROR: DIB_PYTHON_VERSION is '$DIB_PYTHON_VERSION' but needs to be 2 or 3"
     exit 1
 fi
+
+$apt_get install -y $to_install

diskimage_builder/elements/debian-systemd/README.rst

@@ -4,4 +4,7 @@ debian-systemd
 You may want to use `systemd` instead of the classic sysv init system.
 In this case, include this element in your element list.
 
+Note that this works with the ``debian`` element, not the
+``debian-minimal`` element.
+
 .. element_deps::

diskimage_builder/elements/debian-upstart/README.rst

@@ -5,4 +5,7 @@ By default Debian will use sysvinit for booting. If you want to experiment
 with Upstart, or have need of it due to a need for upstart jobs, this
 element will build the image with upstart as the init system.
 
+Note that this works with the ``debian`` element, not the
+``debian-minimal`` element.
+
 .. element_deps::