stx-openstack failed with referenced before assignment

[Hugo Nicodemos Brito]

Brief Description
-----------------
Failing to show the correct exception message duo a variable being referenced before assignment.

Severity
--------
Minor: System/Feature is usable with minor issue

Steps to Reproduce
------------------
Try to install the stx-openstack without an image on the registry

Expected Behavior
------------------
The error message showing the image that failed to download from
the registry.

Actual Behavior
----------------
Application apply aborted duo: `UnboundLocalError: local variable 'target_img_tag' referenced before assignment`

Reproducibility
---------------
Reproducible

System Configuration
--------------------
One node system, Two node system, IPv4

Branch/Pull Time/Commit
-----------------------
N/A

Last Pass
---------
N/A

Timestamp/Logs
--------------
N/A

Test Activity
-------------
N/A

Workaround
----------
N/A

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/config/+/806729

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/config/+/806729
Committed: https://opendev.org/starlingx/config/commit/88cacfc5d582a3bb7524a1a5599b289a52ab1f0d
Submitter: "Zuul (22348)"
Branch: master

commit 88cacfc5d582a3bb7524a1a5599b289a52ab1f0d
Author: Hugo Brito <email address hidden>
Date: Tue Aug 31 08:25:30 2021 -0300

    Fix download image registry exception message

    This changes the variable used on an exception message
    when an image download failed from public/private
    registry. Now, if the exception occurs the variable being
    used has not yet been assigned.

    Closes-Bug: 1942199
    Signed-off-by: Hugo Brito <email address hidden>
    Change-Id: I8d393ae6a6d92fe649d3594cf32ab22988fcde48