[AIRFLOW-3086] Add extras group for google auth to setup.py.

To clarify installation instructions for the google auth backend, add an
install group to `setup.py` that installs dependencies google auth via
`pip install apache-airflow[google_auth]`.

Author: jmcarp

setup.py

@@ -192,6 +192,7 @@ def write_version(filename=os.path.join(*['airflow',
     'pandas-gbq'
 ]
 github_enterprise = ['Flask-OAuthlib>=0.9.1']
+google_auth = ['Flask-OAuthlib>=0.9.1']
 hdfs = ['snakebite>=2.7.8']
 hive = [
     'hmsclient>=0.1.0',
@@ -358,6 +359,7 @@ def do_setup():
             'emr': emr,
             'gcp_api': gcp_api,
             'github_enterprise': github_enterprise,
+            'google_auth': google_auth,
             'hdfs': hdfs,
             'hive': hive,
             'jdbc': jdbc,