Relax Zuul shellcheck lint SC2319 overwrite return value

[Jim Gauld]

Brief Description
-----------------
Zuul tox job now failing for stx-puppet.

In /home/zuul/src/opendev.org/starlingx/stx-puppet/puppet-manifests/src/bin/apply_network_config.sh line 374:
        upgr_bootstrap=$?
                       ^-- SC2319 (warning): This $? refers to a condition, not a command. Assign to a variable to avoid it being overwritten.

Possible fixes:
* Modify the problematic shell code apply_network_config.sh . (not great, the code isn't that bad)
This open review addresses this specifically.
  https://review.opendev.org/c/starlingx/stx-puppet/+/910257

* Pin the shellcheck version:
  passes with: shellcheck-py-0.9.0.6
  fails with: shellcheck-py-0.10.0.1

* Relax the specific checks that are failing:
  # shellcheck disable=SC2119,SC2120
  or, -eSC2319

Severity
--------
Critical: Prevents code submission.

Steps to Reproduce
------------------
Update stx-puppet with a patchset.

Expected Behavior
------------------
Previous codebase should pass existing linters.

Actual Behavior
----------------
Changed upstream version of shellcheck has different behavior for specific tests.

Reproducibility
---------------
100%

System Configuration
--------------------
gerrit review

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

Last Pass
---------
Recent.

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

Test Activity
-------------
Development

Workaround
----------
None.