[AIRFLOW-511] add success/failure callbacks on dag level

[Acehaidrey]

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow 511 issues and references them in the PR title. For example, "[AIRFLOW-511] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-511

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Currently when a dagrun timeout happens there is not an alert sent off on a task failure callback because the way it works is that a timeout just prevents downstream tasks from getting scheduled and it allows the task currently running to finish out. There is not much insight that the dag actually stopped which should definitely be caught.
  Also there are certain callbacks you may want once the entire dag has completed successfully as well, making sense to have both callbacks.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  Tests the callbacks for on success, on failure, and also checks that a dagrun timeout also triggers the callback.

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

[codecov-io]

Codecov Report

Merging #2934 into master will increase coverage by 0.02%.
The diff coverage is 85%.

@@            Coverage Diff             @@
##           master    #2934      +/-   ##
==========================================
+ Coverage   73.46%   73.49%   +0.02%     
==========================================
  Files         160      160              
  Lines       12208    12228      +20     
==========================================
+ Hits         8969     8987      +18     
- Misses       3239     3241       +2

Impacted Files	Coverage Δ
airflow/jobs.py	79.49% <100%> (+0.01%)	⬆️
airflow/models.py	86.97% <84.21%> (+0.02%)	⬆️

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[Acehaidrey]

@Swalloow thanks for pointing out the typo