ansible-role-tripleo-modify-image -- improve failure message when running on the wrong node

In Queens, when somebody executes ansible-role-tripleo-modify-image on the undercloud instead of the overcloud node where the container image can be found. Not sure about other versions of OSP. It would be nice to have a more verbose error message here.

---

(undercloud) [stack@undercloud-1 scripts]$ sudo bash generate_hotfix.sh
Cloning into 'ansible-role-tripleo-modify-image'...
remote: Enumerating objects: 451, done.
remote: Counting objects: 100% (451/451), done.
remote: Compressing objects: 100% (183/183), done.
remote: Total 451 (delta 244), reused 421 (delta 225)
Receiving objects: 100% (451/451), 70.42 KiB | 0 bytes/s, done.
Resolving deltas: 100% (244/244), done.
 [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'

PLAY [localhost] *****************************************************************************************************************************************************************************************************************************

TASK [Gathering Facts] ***********************************************************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:24 -0400 (0:00:00.136) 0:00:00.136 **********
ok: [localhost]

TASK [tripleo-modify-image : Ensure that source_image is defined] ****************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:34 -0400 (0:00:10.786) 0:00:10.922 **********
ok: [localhost] => {
    "changed": false,
    "msg": "All assertions passed"
}

TASK [tripleo-modify-image : Ensure that container_build_tool is correctly set] **************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.157) 0:00:11.080 **********
skipping: [localhost]

TASK [tripleo-modify-image : Set default modified_append_tag] ********************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.129) 0:00:11.209 **********
skipping: [localhost]

TASK [tripleo-modify-image : Ensure that source_image is defined] ****************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.131) 0:00:11.340 **********
ok: [localhost] => {
    "changed": false,
    "msg": "All assertions passed"
}

TASK [tripleo-modify-image : Ensure that container_build_tool is correctly set] **************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.160) 0:00:11.501 **********
skipping: [localhost]

TASK [tripleo-modify-image : Set default modified_append_tag] ********************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.133) 0:00:11.634 **********
skipping: [localhost]

TASK [tripleo-modify-image : Inspect image with Docker] **************************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:35 -0400 (0:00:00.126) 0:00:11.761 **********
ok: [localhost]

TASK [tripleo-modify-image : Set original_user with Docker] **********************************************************************************************************************************************************************************
Tuesday 02 July 2019 15:41:37 -0400 (0:00:01.193) 0:00:12.954 **********
fatal: [localhost]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: list object has no element 0\n\nThe error appears to have been in '/home/stack/openstack_containerized_hotfix/scripts/ansible-role-tripleo-modify-image/tasks/get_original_user.yml': line 11, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n register: source_image_facts\n - name: Set original_user with Docker\n ^ here\n"}

PLAY RECAP ***********************************************************************************************************************************************************************************************************************************
localhost : ok=4 changed=0 unreachable=0 failed=1

Tuesday 02 July 2019 15:41:37 -0400 (0:00:00.142) 0:00:13.097 **********
===============================================================================
Gathering Facts ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 10.79s
tripleo-modify-image : Inspect image with Docker -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1.19s
tripleo-modify-image : Ensure that source_image is defined ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.16s
tripleo-modify-image : Ensure that source_image is defined ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.16s
tripleo-modify-image : Set original_user with Docker ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.14s
tripleo-modify-image : Ensure that container_build_tool is correctly set -------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.13s
tripleo-modify-image : Set default modified_append_tag -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.13s
tripleo-modify-image : Ensure that container_build_tool is correctly set -------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.13s
tripleo-modify-image : Set default modified_append_tag -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 0.13s
(undercloud) [stack@undercloud-1 scripts]$