Move import statment inside sample (GoogleCloudPlatform#1948)

* Move import statment inside sample

Import statement does not appear inside [this sample](https://cloud.google.com/functions/docs/env-var#functions_env_var_access-python).  Makes the published sample misleading because it won't work as-is.

* Linter fix

Author: Anna Piechowski

functions/env_vars/main.py

@@ -12,10 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+
+# [START functions_env_vars]
 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]