[AIRFLOW-5130] Use GOOGLE_APPLICATION_CREDENTIALS constant from library

[mik-laj]

Constant GOOGLE_APPLICATION_CREDENTIALS is used in many places in inconsistent way. I try to organize this small piece of code.

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[codecov-io]

Codecov Report

Merging #5744 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5744      +/-   ##
==========================================
+ Coverage   79.95%   79.97%   +0.01%     
==========================================
  Files         497      498       +1     
  Lines       32078    32151      +73     
==========================================
+ Hits        25649    25713      +64     
- Misses       6429     6438       +9

Impacted Files	Coverage Δ
...irflow/contrib/operators/gcp_container_operator.py	95.83% <100%> (ø)	⬆️
airflow/contrib/hooks/gcp_api_base_hook.py	85.47% <100%> (ø)	⬆️
...ow/contrib/operators/bigquery_to_mysql_operator.py	72.91% <0%> (ø)
airflow/utils/dag_processing.py	58.98% <0%> (+0.18%)	⬆️
airflow/models/taskinstance.py	93.62% <0%> (+0.76%)	⬆️

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