[AIRFLOW-4844] Added optional is_paused_upon_creation argument for da…

[Sinsin1367]

…gs to configure initial pause status upon creation.

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Jira

• My PR addresses the following Airflow-4844

Description

• Here are some details about my PR:

1. Adding the feature to configure each dag object with the initial state of is_paused flag
2. The corresponding changes for python2 branch will be submitted once this review is finalized.

Tests

• My PR adds the following unit tests:

1. Adding unit test to test existing dags in database not being affected with the flag.
2. Adding unit test to test new dags following the flag passed. (there are already unit tests testing backward compatibility of the the code change.)

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

[codecov-io]

Codecov Report

Merging #5482 into v1-10-stable will decrease coverage by <.01%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##           v1-10-stable    #5482      +/-   ##
================================================
- Coverage          79.5%   79.49%   -0.01%     
================================================
  Files               250      250              
  Lines             18264    18267       +3     
================================================
+ Hits              14520    14522       +2     
- Misses             3744     3745       +1

Impacted Files	Coverage Δ
airflow/models/dag.py	93.72% <100%> (+0.02%)	⬆️
airflow/models/taskinstance.py	92.97% <0%> (-0.17%)	⬇️

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[ashb]

Thanks, but don't worry about back-porting. We (the committers) will do that as we prefer to record the commit it is cherry-picked from. I'll deal with getting this in to the release.