[AIRFLOW-4482] Add execution_date to trigger DAG run API response

[shuva10v]

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Jira

• My PR addresses the following Airflow Jira AIRFLOW-4482 issues and references them in the PR title.

Description

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

We are using experimental REST API for automating Airflow from Jenkins and our workflow looks like this:

• Jenkins job triggers DAG run (using POST /api/experimental/dags/<dag_id>/dag_runs)
• Airflow API returns response like this: 

{"message":"Created <DagRun dag_id @ 2019-05-08 11:42:43+00:00: run_id, externally triggered: True>"}

• Jenkins job parses the response for execution_date (2019-05-08T11:42:43 for response above)
• Jenkins job further uses execution_date to check DAG run state using POST /api/experimental/dags/<dag_id>/dag_runs/<execution_date>
• Also Jenkins job generates DAG run UI link

Here is the code - how it works.

Actually it is not a good idea to parse message string for execution_date. So it is proposed to add execution_date in DAG run trigger API response as a field. It makes it possible to get execution_date directly from the response without dirty and unpredictable parsing. After this improvement API response will be like this:

{'execution_date': '2019-05-08T07:03:09+00:00', 'message': 'Created <DagRun dag_id @ 2019-05-08 07:03:09+00:00: manual__2019-05-08T07:03:09+00:00, externally triggered: True>'} 

Tests

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

Asserts added for test_endpoints.test_trigger_dag

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.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[XD-DENG]

Thanks @shuva10v A useful feature to me.

I have restarted the failing test which may be due to transient error.

[codecov-io]

Codecov Report

Merging #5260 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5260      +/-   ##
==========================================
+ Coverage   78.39%   78.66%   +0.26%     
==========================================
  Files         470      470              
  Lines       30013    30013              
==========================================
+ Hits        23528    23609      +81     
+ Misses       6485     6404      -81

Impacted Files	Coverage Δ
airflow/www/api/experimental/endpoints.py	88.77% <100%> (ø)	⬆️
airflow/hooks/dbapi_hook.py	88.69% <0%> (+0.86%)	⬆️
airflow/models/connection.py	65.73% <0%> (+1.12%)	⬆️
airflow/hooks/hive_hooks.py	75% <0%> (+1.86%)	⬆️
airflow/utils/sqlalchemy.py	80.95% <0%> (+4.76%)	⬆️
airflow/operators/mysql_operator.py	100% <0%> (+100%)	⬆️
airflow/operators/mysql_to_hive.py	100% <0%> (+100%)	⬆️

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