[AIRFLOW-3540] Warn if old airflow.cfg file is found

[ashb]

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Jira

• https://issues.apache.org/jira/browse/AIRFLOW-3540

Description

• People might have been caught by this situation of having (a dual config file) without even knowning it. To make it obvious for those people we should say if we found the old config file even if we don't read from it.

  Note: Against v1-10-stable branch, not master.

  A DeprecationWarning may not actually be the right thing here, as we aren't reading from it. Anyone have any thoughts?

Tests

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

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

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #5006 into v1-10-stable will decrease coverage by <.01%.
The diff coverage is 66.66%.

@@               Coverage Diff               @@
##           v1-10-stable   #5006      +/-   ##
===============================================
- Coverage            78%     78%   -0.01%     
===============================================
  Files               226     226              
  Lines             17626   17629       +3     
===============================================
+ Hits              13749   13751       +2     
- Misses             3877    3878       +1

Impacted Files	Coverage Δ
airflow/configuration.py	89% <66.66%> (-0.23%)	⬇️

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