chore(celery) & docs(celery): update celery config variables names

[artemonsh]

SUMMARY

1. Class attributes of CeleryConfig in docker/pythonpath_dev/superset_config.py are updated due to the lowercase settings introduced in version 4.0.
2. Variable worker_log_level contained in config.py is not mentioned anywhere in the celery documentation, therefore should be removed. It can be replaced with CELERYD_LOG_LEVEL or could be removed. To configure logging level option -l is used as mentioned here. This option is already used in docker-bootstrap.sh:

   superset/docker/docker-bootstrap.sh

   Line 40 in b954f8f

   celery --app=superset.tasks.celery_app:app worker -Ofair -l INFO

   hence its usage is excessive in both config.py and superset_config.py and could be removed.
3. For some reason optimization option -O was glued to its value fair like this: -Ofair. Suggested change unglues them in docker-bootstrap.sh and CONTRIBUTING.md.

ADDITIONAL INFORMATION

• Has associated issue: docker/pythonpath_dev/superset_config.py celery args are outdated #20808
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #22499 (c9b73df) into master (b954f8f) will increase coverage by 0.20%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #22499      +/-   ##
==========================================
+ Coverage   66.90%   67.10%   +0.20%     
==========================================
  Files        1851     1866      +15     
  Lines       70697    71337     +640     
  Branches     7764     7764              
==========================================
+ Hits        47300    47873     +573     
- Misses      21375    21442      +67     
  Partials     2022     2022              

Flag	Coverage Δ
hive	52.63% <ø> (+0.17%)	⬆️
mysql	78.10% <ø> (+0.12%)	⬆️
postgres	78.17% <ø> (+0.12%)	⬆️
presto	52.53% <ø> (+0.17%)	⬆️
python	81.40% <ø> (+0.15%)	⬆️
sqlite	76.56% <ø> (+0.05%)	⬆️
unit	51.51% <ø> (+0.53%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/config.py	91.64% <ø> (+0.15%)	⬆️
superset/db_engine_specs/bigquery.py	69.60% <0.00%> (-12.09%)	⬇️
superset/dashboards/filter_state/commands/utils.py	80.00% <0.00%> (-10.00%)	⬇️
...uperset/dashboards/filter_state/commands/delete.py	91.30% <0.00%> (-4.35%)	⬇️
superset/databases/commands/update.py	75.86% <0.00%> (-4.14%)	⬇️
...uperset/dashboards/filter_state/commands/update.py	96.66% <0.00%> (-3.34%)	⬇️
superset/views/datasource/schemas.py	97.22% <0.00%> (-2.78%)	⬇️
superset/dao/base.py	92.78% <0.00%> (-2.62%)	⬇️
superset/databases/commands/test_connection.py	97.43% <0.00%> (-1.24%)	⬇️
superset/sql_lab.py	81.13% <0.00%> (-1.00%)	⬇️
... and 47 more

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[villebro]

I agree with these changes, thanks for the cleanup.

[dpgaspar]

LGTM, thank you!