[AIRFLOW-1457] fix user-defined log import

[ahvigil]

We can use json.load to import a user defined json config for passing into logging.config.dictConfig

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the 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-1457

Description

• Here are some details about my PR, including screenshots of any UI changes:

user-defined logging configuration provided as part of AIRFLOW-1457 is broken- current code tries to import a string value pulled from the config. This always fails and results in the default config being loaded.

Since contents of logging_config_path will get passed to logging.config.DictConfig as a dictionary, it should be json de/serializable and so we can use json.load to read the file instead.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  I can add tests at some point if you want, but settings.py currently does not have any unit tests and this is a pretty core functionality that is broken so I really just want to get it working :)

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
  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"

[criccomini]

The JIRA looks different from the PR

[ahvigil]

argh, i see now this is being addressed now in #2631

[codecov-io]

Codecov Report

Merging #2648 into master will decrease coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2648      +/-   ##
==========================================
- Coverage   71.75%   71.75%   -0.01%     
==========================================
  Files         152      152              
  Lines       11749    11751       +2     
==========================================
+ Hits         8431     8432       +1     
- Misses       3318     3319       +1

Impacted Files	Coverage Δ
airflow/settings.py	88.29% <66.66%> (-0.84%)	⬇️

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