Prepare backport packages for providers: google

[potiuk]

We would like to release the google backport package as described in using hooks and operators from master in Airflow 1.10

The providers package needs to have set of system tests that need to test operators/hooks/sensors released in the package.

How to prepare system tests is described in System tests.

• [example dags] must exist in airflow/providers/google package
• the example dags should be ready to execute them e2e
• the example dags should be configurable via environment variables
• pytest test running the DAGs must exist in tests/providers/google package
• the example dags should authenticate using
  • forwarded credentials or
  • the @pytest.mark.credential_file annotation
• The BACKPORT_README.md file must describe release notes for the package

Current status of backport packages is described in Backport packages status page