Change phase to check for dracut-regenerate in iscsi-boot

There is an use of get_image_element_array on the environment.d
phase, for the iscsi-boot element.
This function is not available on that step. So moving the check
at next step, extra-data-d, where it is available.

Change-Id: I89cfe565492142c2f7962109360fcbcebadfd469

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

@@ -1,10 +1,4 @@
 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

diskimage_builder/elements/iscsi-boot/extra-data.d/50-check-dracut-regenerate

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+if [ ${DIB_DEBUG_TRACE:-1} -gt 0 ]; then
+    set -x
+fi
+set -eu
+set -o pipefail
+
+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
+
+fi