[AIRFLOW-1619] Add poll_sleep parameter to GCP dataflow operator

[TJBIII]

Dear Airflow maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the 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-1619

Description

• Here are some details about my PR, including screenshots of any UI changes: Add parameter to set the time in seconds to sleep between polling GCP for the dataflow job status while the job is in the JOB_STATE_RUNNING state. Allows for a more reasonable poll interval than the previously hard coded 10 seconds for long running jobs.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason: Added to tests/contrib/operators/test_dataflow_operator.py

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"

[codecov-io]

Codecov Report

Merging #2612 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2612      +/-   ##
==========================================
- Coverage    70.7%   70.69%   -0.01%     
==========================================
  Files         150      150              
  Lines       11630    11630              
==========================================
- Hits         8223     8222       -1     
- Misses       3407     3408       +1

Impacted Files	Coverage Δ
airflow/models.py	87.1% <0%> (-0.05%)	⬇️

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[criccomini]

+1 LGTM