Add iscsi-boot element for CentOS images

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

Tested on Fujitsu Server and boot with BIOS and UEFI mode successfully.
- Tested Boot-From-Volume + EFI for centos7 with following elements:
  "centos7 vm devuser cloud-init-datasources dhcp-all-interfaces
   iscsi-boot dracut-regenerate block-device-efi"

Co-authored-By: Nguyen Van Trung <trungnv@vn.fujitsu.com>

Change-Id: Ia1f23d722dced6f254fd7aee86abe8066a72fa42

diskimage_builder/elements/iscsi-boot/README.rst

@@ -3,11 +3,18 @@ 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.
+can configure Ubuntu/Debian images and CentOS7 images.
 
 It performs the following actions:
 
+For Ubuntu/Debian images:
   * 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.
+
+For CentOS7 images:
+  * Required ``dracut-regenerate`` element when performs ``disk-image-create``.
+  * Updates ``network`` and ``iscsi`` into ``dracut-regenerate`` during
+    pre-installs.
+  * Updates ``GRUB_CMDLINE_LINUX_DEFAULT`` into ``/etc/default/grub``.

diskimage_builder/elements/iscsi-boot/environment.d/open-iscsi-config

@@ -0,0 +1,14 @@
+if [[ $DISTRO_NAME = "centos7" ]]; then
+    eval declare -A image_elements=($(get_image_element_array))
+    found=0
+    for i in "${!image_elements[@]}"; do
+        [ "$i" = "dracut-regenerate" ] && found=1
+    done
+    [ "$found" = 0 ] && echo "The dracut-regenerate element is required!" && exit 1
+
+    DIB_DRACUT_ENABLED_MODULES+="
+- name: network
+- name: iscsi
+    "
+    export DIB_DRACUT_ENABLED_MODULES
+fi

diskimage_builder/elements/iscsi-boot/finalise.d/51-open-iscsi-config

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
+
+set -eu
+set -o pipefail
+
+if [[ $DISTRO_NAME = "centos7" ]]; then
+    sed -i -E 's/(GRUB_CMDLINE_LINUX_DEFAULT=")(.+)(")/\1\2 rd.iscsi.firmware=1"/' /etc/default/grub
+    grub2-mkconfig -o /boot/grub2/grub.cfg
+fi