Ensure machine-id is not included in images

Two bugs are addressed.

1) The sysprep element was broken in that it only truncates
   /etc/machine-id, but not /var/lib/dbus/machine-id. systemd will
   not generate a new machine-id if /var/lib/dbus/machine-id is
   present[1], it will simply copy it to /etc/machine-id.

   We observed machine-ids being packaged in /var/lib/dbus/machine-id
   on several distros: Ubuntu Bionic, Fedora 29, Debian Stretch.

   CentOS 7 and Ubuntu Xenial do not contain packaged machine-id as
   far as I can tell.

   All test builds were performed using -minimal elements.

2) A second bug existed where debian-minimal did not run the sysprep
   element at all, so a stretch image I tested contained a populated
   /etc/machine-id AND a populated /var/lib/dbus/machine-id.

[1] https://www.freedesktop.org/software/systemd/man/machine-id.html#Initialization

Change-Id: Ibb28b6e90d966a845de38a2cd5a1e8babd2604bc

diskimage_builder/elements/debootstrap/element-deps

@@ -1,4 +1,4 @@
 dpkg
 modprobe
 pkg-map
-
+sysprep

diskimage_builder/elements/sysprep/finalise.d/01-clear-machine-id

@@ -9,3 +9,7 @@ set -o pipefail
 if [ -e /etc/machine-id ]; then
     > /etc/machine-id
 fi
+
+if [ -e /var/lib/dbus/machine-id ]; then
+    > /var/lib/dbus/machine-id
+fi

releasenotes/notes/sysprep-fixes-8890b968a8fa7ac1.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    The sysprep element has been fixed to truncate ``/var/lib/dpkg/machine-id``
+    (``/etc/machine-id`` was already being truncated). This ensures a
+    machine-id is not packaged in the image and systemd will be forced to
+    generate a new one upon first boot.
+  - |
+    The sysprep element is added as a dependency to the ``debootstrap`` element to
+    ensure that it runs on all Debian builds, including debian-minimal based
+    images.