[AIRFLOW-3402] Port PR #4247 to 1.10-test

[kppullin]

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-3402

Description

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

This is a manual port of AIRFLOW-3402 (PR #4247) to the v1-10-test branch, as cherry picking lead to conflicts.

Tests

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

Tests ported from original PR.

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"

Code Quality

• Passes flake8

(note, flake8 tests appear broken in the current HEAD of v1.10-test, but no issues related to this change).

[kppullin]

@kaxil - Per your request in #4247, I've done a manual merge to the v1-10-test branch.

As noted above, the CI test run is currently failed due to an unrelated, existing issue with the flake8 tests. All other tests are passing on the CI runs from my fork (https://travis-ci.org/kppullin/airflow/builds/476172231).