[AIRFLOW-2401] Add syntax to use variables from Jinja template

[moe-nadal-ck]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA 2401 issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-2401

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
  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"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[moe-nadal-ck]

Simple change in the documentation for the Variable usage from jinja template

[codecov-io]

Codecov Report

Merging #2847 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #2847     +/-   ##
=========================================
- Coverage   74.21%   74.01%   -0.2%     
=========================================
  Files         193      160     -33     
  Lines       14433    12131   -2302     
=========================================
- Hits        10711     8979   -1732     
+ Misses       3722     3152    -570

Impacted Files	Coverage Δ
airflow/operators/redshift_to_s3_operator.py	0% <0%> (-100%)	⬇️
airflow/hooks/mssql_hook.py	6.66% <0%> (-66.67%)	⬇️
airflow/hooks/hdfs_hook.py	32.5% <0%> (-60%)	⬇️
airflow/operators/hive_operator.py	41.02% <0%> (-32.31%)	⬇️
airflow/utils/log/s3_task_handler.py	80.3% <0%> (-16.84%)	⬇️
airflow/bin/cli.py	53.94% <0%> (-7.81%)	⬇️
airflow/utils/decorators.py	87.5% <0%> (-4.17%)	⬇️
airflow/www/app.py	96.55% <0%> (-3.45%)	⬇️
airflow/operators/check_operator.py	54.03% <0%> (-3.45%)	⬇️
airflow/jobs.py	79.89% <0%> (-3.2%)	⬇️
... and 85 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a15bd80...8ebef4a. Read the comment docs.

[r39132]

Please still add a JIRA so that we can generate clean-looking release notes. Follow the contributor guidelines.

[moe-nadal-ck]

@r39132 Done!

[r39132]

+1 thx

[r39132]

FYI.. you should not make the JIRA resolved yourself -- it's automatically marked Resolved when committers/maintainers merge your fix.