Clear /etc/machine-id to avoid duplicate machine-ids

Deploying many nodes with the generated image shouldn't have the same
/etc/machine-id so clearing it and letting systemd generate a new
id upon first boot seems to be the best way to achieve this.

Change-Id: I73d0577d31464521b3989312fd9d982a1312a268
Closes-bug: 1707526
Closes-bug: 1672461

diskimage_builder/elements/debian/element-deps

@@ -1,2 +1,3 @@
 debian-minimal
 openssh-server
+sysprep

diskimage_builder/elements/opensuse-minimal/element-deps

@@ -1 +1,2 @@
+sysprep
 zypper-minimal

diskimage_builder/elements/opensuse/element-deps

@@ -1,3 +1,4 @@
 cache-url
 package-installs
+sysprep
 zypper

diskimage_builder/elements/redhat-common/element-deps

@@ -1 +1,2 @@
 package-installs
+sysprep

diskimage_builder/elements/sysprep/README.rst

@@ -0,0 +1,5 @@
+sysprep
+=======
+
+This element holds configuration and scripts that are common for all
+distributions.

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

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+if [ -e /etc/machine-id ]; then
+    > /etc/machine-id
+fi

diskimage_builder/elements/ubuntu-common/element-deps

@@ -0,0 +1 @@
+sysprep

releasenotes/notes/sysprep-f3fd036bc1d2c405.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    Adds sysprep element included by all systemd distros
+fixes:
+  - |
+    Adds default sysprep element clearing /etc/machine-id which
+    prevents duplicated /etc/machine-id by forcing systemd to
+    generate a new id for each booted system.