[AIRFLOW-6060] Improve conf_vars context manager

[turbaszek]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-6060

Description

• Here are some details about my PR, including screenshots of any UI changes:
  This commit adds try / finally clause to conf_vars context
  manager to assure that initail values are reseted in case
  of an exception in yield.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6658 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6658      +/-   ##
==========================================
- Coverage   83.82%   83.48%   -0.34%     
==========================================
  Files         672      672              
  Lines       37560    37560              
==========================================
- Hits        31483    31358     -125     
- Misses       6077     6202     +125

Impacted Files	Coverage Δ
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
airflow/configuration.py	89.13% <0%> (-3.63%)	⬇️
airflow/jobs/backfill_job.py	89.9% <0%> (-1.53%)	⬇️

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[mik-laj]

Can you add a more descriptive title e.g. Handle exceptions in conf_vars context manager (46 characters)?