Add iscsi-boot element

This patch adds an element that handles the configuration for
creating a disk capable of being a remote root filesystem through
iSCSI on Ubuntu and Debian images.

Change-Id: Ibf9e39d2bdab530106015f156d23d28029d12b0d
Closes-bug: #1716794

diskimage_builder/elements/iscsi-boot/README.rst

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+==========
+iscsi-boot
+==========
+Handles configuration for the disk to be capable of serving as
+a remote root filesystem through iSCSI. Currently, this element
+only configures Ubuntu/Debian images.
+
+It performs the following actions:
+
+* Installs the ``open-iscsi`` package.
+* Creates the ``etc/iscsi/iscsi.initramfs`` configuration file and sets
+  ``ISCSI_AUTO=true`` within it.
+* Updates the initramfs to apply the changes.

diskimage_builder/elements/iscsi-boot/element-deps

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+package-installs

diskimage_builder/elements/iscsi-boot/package-installs.yaml

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+open-iscsi:
+  phase: install.d

diskimage_builder/elements/iscsi-boot/post-install.d/open-iscsi-config

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+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+
+set -eu
+set -o pipefail
+
+if [[ $DISTRO_NAME = "ubuntu" || $DISTRO_NAME = "debian" ]]; then
+    echo "ISCSI_AUTO=true" > /etc/iscsi/iscsi.initramfs
+    update-initramfs -u
+fi