Added sample for Using Environment Variables. (#1653)

* Added sample for Using Environment Variables.

* Added default value.

Author: chenyumic

functions/env_vars/README.md

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+<img src="https://avatars2.githubusercontent.com/u/2810941?v=3&s=96" alt="Google Cloud Platform logo" title="Google Cloud Platform" align="right" height="96" width="96"/>
+
+# Google Cloud Functions - Using Environment Variables sample
+
+See:
+* [Cloud Functions Using Environment Variables tutorial][tutorial]
+* [Cloud Functions Using Environment Variables sample source code][code]
+ [tutorial]: https://cloud.google.com/functions/docs/env-var#functions_env_var-python
+[code]: main.py

functions/env_vars/main.py

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+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+
+# [START functions_env_vars]
+def env_vars(request):
+    return os.environ.get('FOO', 'Specified environment variable is not set.')
+# [END functions_env_vars]

functions/env_vars/main_test.py

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+# Copyright 2018 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the 'License');
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an 'AS IS' BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+
+import flask
+import pytest
+
+import main
+
+
+# Create a fake "app" for generating test request contexts.
+@pytest.fixture(scope="module")
+def app():
+    return flask.Flask(__name__)
+
+
+def test_env_vars(app):
+    with app.test_request_context():
+        os.environ['FOO'] = 'bar'
+        res = main.env_vars(flask.request)
+        assert res == 'bar'