Set container_images_file for containerized undercloud.

While performing upgrade to containerized undercloud it's needed
to specify file with docker images to be used.
Generating this file is out of scope for tripleo-upgrade.

Change-Id: I626366470673001e5d86cb9756e994e001751e17

tasks/upgrade/main.yml

@@ -13,6 +13,20 @@
         chdir: "{{working_dir}}"
       when: upgrade_workarounds
 
+    - name: check customized {{ uc_containers_prepare_file }} exists
+      stat:
+        path: "{{ working_dir }}/{{ uc_containers_prepare_file }}"
+      register: custom_uc_containers
+
+    - name: set containers file for undercloud
+      lineinfile:
+        path: "{{ undercloud_conf }}"
+        regexp: '^(container_images_file)(.*)'
+        line: "container_images_file = {{ working_dir }}/{{ uc_containers_prepare_file }}"
+        insertafter: ^\[DEFAULT\]
+      when:
+        - custom_uc_containers.stat.exists
+
     - name: upgrade undercloud
       shell: |
         set -o pipefail