[AIRFLOW-1837] Respect task start_date when different from dag's

[dima-asana]

Currently task instances get created and scheduled based on the DAG's
start date rather than their own. This commit adds a check before
creating a task instance to see that the start date is not after
the execution date.

Make sure you have checked all steps below.

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-1837

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Currently task instances get created and scheduled based on the DAG's
  start date rather than their own. This PR adds a check before
  creating a task instance to see that the start date is not after
  the execution date.

Tests

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

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

• [N/A] In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.

Code Quality

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