Upgraded deploys don't support old Barbican registry secrets

[João Victor Portal]

Brief Description
-----------------
After upgrading StarlingX to a newer version, the system cannot retrieve the registries username and passwords from Barbican secrets of content type "text/plain", throwing the error "'str' object has no attribute 'decode'" in "sysinv.log".

Severity
--------
Critical

Steps to Reproduce
------------------
Upgrade StarlingX to a newer version.

Expected Behavior
------------------
The images needed by applications to be installed on the system are downloaded from registry.

Actual Behavior
----------------
No image can be downloaded from registry.

Reproducibility
---------------
100% reproducible.

System Configuration
--------------------
Any.

Branch/Pull Time/Commit
-----------------------
NA.

Last Pass
---------
NA.

Timestamp/Logs
--------------
NA.

Test Activity
-------------
Regression Testing.

Workaround
----------
NA.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/config/+/876781

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/config/+/876781
Committed: https://opendev.org/starlingx/config/commit/f5a8ba9f22cc599a645e744d5ddbb56cbaed405d
Submitter: "Zuul (22348)"
Branch: master

commit f5a8ba9f22cc599a645e744d5ddbb56cbaed405d
Author: Joao Victor Portal <email address hidden>
Date: Tue Mar 7 17:53:54 2023 -0300

    Support old Barbican registry secrets

    Older versions of StarlingX created Barbican registry secrets as
    "text/plain", that are retrieved as "string" in Python 3. This change
    adds support to these secrets, that may be used in upgraded systems. In
    non-upgraded newer systems, these secrets are created as
    "application/octet-stream", being retrieved as "bytes" to be decoded
    using UTF-8 in Python 3.

    Test Plan:

    PASS: Check that the system can retrive the registries usernames and
    passwords when the secrets have the content type as "text/plain".

    PASS: Check that the system can retrive the registries usernames and
    passwords when the secrets have the content type as
    "application/octet-stream".

    Closes-Bug: 2009631
    Signed-off-by: Joao Victor Portal <email address hidden>
    Change-Id: I5a71239b09ef1124449dc66f86ef790e1f23222c