[AIRFLOW-5195] Move GCP Dataflow to core

[turbaszek]

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-5195

Description

• Here are some details about my PR, including screenshots of any UI changes:
  This PR moves GCP Dataflow from contrib to core. For more information check AIP-21.

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 (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

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[turbaszek]

Depends on #5791.

[mik-laj]

======================================================================
45) ERROR: testSuccessfulRun (tests.contrib.utils.test_mlengine_operator_utils.CreateEvaluateOpsTest)
----------------------------------------------------------------------
   Traceback (most recent call last):
    tests/contrib/utils/test_mlengine_operator_utils.py line 100 in testSuccessfulRun
      with patch('airflow.contrib.operators.dataflow_operator.'
    /usr/local/lib/python3.6/unittest/mock.py line 1247 in __enter__
      original, local = self.get_original()
    /usr/local/lib/python3.6/unittest/mock.py line 1221 in get_original
      "%s does not have the attribute %r" % (target, name)
   AttributeError: <module 'airflow.contrib.operators.dataflow_operator' from '/opt/airflow/airflow/contrib/operators/dataflow_operator.py'> does not have the attribute 'DataFlowHook'
   -------------------- >> begin captured logging << --------------------
   py.warnings: WARNING: /opt/airflow/airflow/contrib/operators/dataflow_operator.py:32: DeprecationWarning: This module is deprecated. Please use `airflow.gcp.operators.dataflow`.
     DeprecationWarning,
   
   --------------------- >> end captured logging << ---------------------