fix airflow-webserer startup errors (v1-10-test)

[Kurganov]

fix airflow-webserer startup errors

Jul 29 17:00:01 localhost sh: Traceback (most recent call last):
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/bin/airflow", line 37, in <module>
Jul 29 17:00:01 localhost sh: args.func(args)
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/lib/python3.6/site-packages/airflow/utils/cli.py", line 76, in wrapper
Jul 29 17:00:01 localhost sh: return f(*args, **kwargs)
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/lib/python3.6/site-packages/airflow/bin/cli.py", line 1076, in webserver
Jul 29 17:00:01 localhost sh: app = cached_app_rbac(None) if settings.RBAC else cached_app(None)
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/lib/python3.6/site-packages/airflow/www/app.py", line 234, in cached_app
Jul 29 17:00:01 localhost sh: app = create_app(config, testing)
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/lib/python3.6/site-packages/airflow/www/app.py", line 89, in create_app
Jul 29 17:00:01 localhost sh: from airflow.www import views
Jul 29 17:00:01 localhost sh: File "/home/airflow/venv/lib/python3.6/site-packages/airflow/www/views.py", line 99, in <module>
Jul 29 17:00:01 localhost sh: login_required = airflow.login.login_required
Jul 29 17:00:01 localhost sh: AttributeError: module 'airflow.contrib.auth.backends.kerberos_auth' has no attribute 'login_required'
Jul 29 17:00:01 localhost systemd: airflow-webserver.service: main process exited, code=exited, status=1/FAILURE

if airflow.api.auth.backend.kerberos_auth is enabled

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, pylint and type annotations). Our pre-commits will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example DAG that shows how users should use it.
• Consider using Breeze environment for testing locally, it’s a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: [email protected]
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[mik-laj]

@Kurganov Can I ask for a review?

[Kurganov]

done

[kaxil]

Has been cherry-picked to v1-10-test: 8e11423