[AIRFLOW-3987] Unify GCP's Connection IDs

[ryanyuan]

Unify GCP's Connection IDs

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Jira

• My PR addresses the following Airflow-3987 issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"

Description

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

Tests

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

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.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4818 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4818      +/-   ##
==========================================
- Coverage   75.74%   75.74%   -0.01%     
==========================================
  Files         458      458              
  Lines       29850    29849       -1     
==========================================
- Hits        22609    22608       -1     
  Misses       7241     7241

Impacted Files	Coverage Δ
airflow/contrib/sensors/bigquery_sensor.py	0% <ø> (ø)	⬆️
...rflow/contrib/operators/bigquery_check_operator.py	0% <ø> (ø)	⬆️
airflow/contrib/operators/gcs_to_bq.py	0% <ø> (ø)	⬆️
airflow/contrib/hooks/bigquery_hook.py	60.89% <ø> (ø)	⬆️
airflow/contrib/operators/bigquery_get_data.py	0% <ø> (ø)	⬆️
airflow/contrib/operators/bigquery_operator.py	93.57% <ø> (ø)	⬆️
airflow/contrib/operators/bigquery_to_bigquery.py	0% <ø> (ø)	⬆️
...ontrib/operators/bigquery_table_delete_operator.py	0% <ø> (ø)	⬆️
airflow/contrib/operators/gcs_to_s3.py	96.77% <ø> (ø)	⬆️
airflow/utils/db.py	90.19% <ø> (-0.1%)	⬇️
... and 2 more

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[potiuk]

Superb!

[ryanyuan]

PTAL @kaxil

[kaxil]

I'll have a look at this tonight or at most by the end of this week :)

[Fokko]

@ryanyuan Could you resolve the conflicts? :-)

[kaxil]

Can we rephrase the last line ?

[kaxil]

Minor nits. LGTM

[ryanyuan]

@potiuk @kaxil Thanks for the review.

[kaxil]

LGTM. Thanks @ryanyuan