Add ubuntu-systemd-container operating-system element

In order to allow the simple preparation of base images which
can be used for LXC/nspawn machine containers, we add this
element.

Containers inherit a kernel from the host, so there is no need
to build a kernel into the image. All the element needs is a
base init system which, in this case, is systemd.

Change-Id: I45651de2aa1b19bdeee301094f0bdffdd0a3b45c

diskimage_builder/elements/ubuntu-systemd-container/README.rst

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+========================
+ubuntu-systemd-container
+========================
+
+The ``ubuntu-systemd-container`` element uses debootstrap for generating
+a minimal image for use by machine containers. In contrast the ``ubuntu``
+element uses the cloud-image as the initial base and the ``ubuntu-minimal``
+builds an image to be used for hosts.
+
+By default this element creates the latest LTS release.  The exact
+setting can be found in the ``ubuntu-common`` element's ``environment.d``
+directory in the variable ``DIB_RELEASE``.  If a different release of
+Ubuntu should be created, the variable ``DIB_RELEASE`` can be set
+appropriately.
+
+Note that this element installs ``systemd-sysv`` as the init system for
+18.04+.
+
+.. element_deps::

diskimage_builder/elements/ubuntu-systemd-container/element-deps

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+debootstrap
+ubuntu-common

diskimage_builder/elements/ubuntu-systemd-container/element-provides

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+operating-system

diskimage_builder/elements/ubuntu-systemd-container/root.d/75-ubuntu-minimal-baseinstall

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+#!/bin/bash
+# Copyright (c) 2014 Hewlett-Packard Development Company, L.P.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+# implied.
+#
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+# We should manage this in a betterer way
+sudo bash -c "cat << EOF >$TARGET_ROOT/etc/apt/sources.list
+deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE ${DIB_DEBIAN_COMPONENTS//,/ }
+deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-updates ${DIB_DEBIAN_COMPONENTS//,/ }
+deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-backports ${DIB_DEBIAN_COMPONENTS//,/ }
+deb $DIB_DISTRIBUTION_MIRROR $DIB_RELEASE-security ${DIB_DEBIAN_COMPONENTS//,/ }
+EOF"
+
+sudo mount -t proc none $TARGET_ROOT/proc
+sudo mount -t sysfs none $TARGET_ROOT/sys
+trap "sudo umount $TARGET_ROOT/proc; sudo umount $TARGET_ROOT/sys" EXIT
+
+apt_get="sudo chroot $TARGET_ROOT /usr/bin/apt-get" # dib-lint: safe_sudo
+
+# Need to update to retrieve the signed Release file
+$apt_get update
+
+$apt_get clean
+$apt_get dist-upgrade -y
+
+to_install=""
+
+# pre-bionic (18.04) 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.
+if [ $DIB_RELEASE != "trusty" ] && [ $DIB_RELEASE != "xenial" ]; then
+    to_install+="systemd-sysv "
+fi
+
+# default required
+to_install+="sudo " # dib-lint: safe_sudo
+
+if [ "$DIB_PYTHON_VERSION" == "2" ]; then
+    to_install+="python "
+elif [ "$DIB_PYTHON_VERSION" == "3" ]; then
+    to_install+="python3 "
+else
+    echo "ERROR: DIB_PYTHON_VERSION is '$DIB_PYTHON_VERSION' but needs to be 2 or 3"
+    exit 1
+fi
+
+$apt_get install -y $to_install

diskimage_builder/elements/ubuntu-systemd-container/test-elements/bionic-build-succeeds/README.rst

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+Verify we can build a ubuntu-systemd-container image.

diskimage_builder/elements/ubuntu-systemd-container/test-elements/bionic-build-succeeds/element-deps

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+openstack-ci-mirrors

diskimage_builder/elements/ubuntu-systemd-container/test-elements/bionic-build-succeeds/environment.d/10-set-distro.bash

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+export DIB_RELEASE=bionic

diskimage_builder/elements/ubuntu-systemd-container/test-elements/bionic-build-succeeds/test-output-formats

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+tar

diskimage_builder/elements/ubuntu-systemd-container/test-elements/trusty-build-succeeds/README.rst

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+Verify we can build a ubuntu-systemd-container image.

diskimage_builder/elements/ubuntu-systemd-container/test-elements/trusty-build-succeeds/element-deps

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+openstack-ci-mirrors

diskimage_builder/elements/ubuntu-systemd-container/test-elements/trusty-build-succeeds/environment.d/10-set-distro.bash

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+export DIB_RELEASE=trusty

diskimage_builder/elements/ubuntu-systemd-container/test-elements/xenial-build-succeeds/README.rst

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+Verify we can build a ubuntu-systemd-container image.

diskimage_builder/elements/ubuntu-systemd-container/test-elements/xenial-build-succeeds/element-deps

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+openstack-ci-mirrors

diskimage_builder/elements/ubuntu-systemd-container/test-elements/xenial-build-succeeds/environment.d/10-set-distro.bash

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+export DIB_RELEASE=xenial

diskimage_builder/elements/ubuntu-systemd-container/test-elements/xenial-build-succeeds/test-output-formats

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+tar