[AIRFLOW-1045] Make log level configurable via airflow.cfg

[sekikn]

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.
  • https://issues.apache.org/jira/browse/AIRFLOW-1045

Description

• Here are some details about my PR:

For now, changing log level needs to modify settings.py directly.
It's inconvenient. This PR makes it configurable via airflow.cfg.

Tests

• My PR adds the following unit tests:

core:LogTest

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"

[mention-bot]

@sekikn, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mistercrunch, @saguziel and @bolkedebruin to be potential reviewers.

[codecov-io]

Codecov Report

Merging #2191 into master will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##           master   #2191      +/-   ##
=========================================
- Coverage   67.24%   67.2%   -0.05%     
=========================================
  Files         142     142              
  Lines       10823   10833      +10     
=========================================
+ Hits         7278    7280       +2     
- Misses       3545    3553       +8

Impacted Files	Coverage Δ
airflow/settings.py	88.88% <66.66%> (-4.07%)	⬇️
airflow/jobs.py	72.94% <0%> (-0.42%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d8c0f59...ec22b63. Read the comment docs.

[bolkedebruin]

Great! LGTM