Update all generated readme auth instructions [(#1121)](GoogleCloudPlatform/python-docs-samples#1121)

Change-Id: I03b5eaef8b17ac3dc3c0339fd2c7447bd3e11bd2

Author: Jon Wayne Parrott

samples/snippets/README.rst

@@ -18,34 +18,12 @@ Setup
 Authentication
 ++++++++++++++
 
-Authentication is typically done through `Application Default Credentials`_,
-which means you do not have to change the code to authenticate as long as
-your environment has credentials. You have a few options for setting up
-authentication:
+This sample requires you to have authentication setup. Refer to the
+`Authentication Getting Started Guide`_ for instructions on setting up
+credentials for applications.
 
-#. When running locally, use the `Google Cloud SDK`_
-
-    .. code-block:: bash
-
-        gcloud auth application-default login
-
-
-#. When running on App Engine or Compute Engine, credentials are already
-   set-up. However, you may need to configure your Compute Engine instance
-   with `additional scopes`_.
-
-#. You can create a `Service Account key file`_. This file can be used to
-   authenticate to Google Cloud Platform services from any environment. To use
-   the file, set the ``GOOGLE_APPLICATION_CREDENTIALS`` environment variable to
-   the path to the key file, for example:
-
-    .. code-block:: bash
-
-        export GOOGLE_APPLICATION_CREDENTIALS=/path/to/service_account.json
-
-.. _Application Default Credentials: https://cloud.google.com/docs/authentication#getting_credentials_for_server-centric_flow
-.. _additional scopes: https://cloud.google.com/compute/docs/authentication#using
-.. _Service Account key file: https://developers.google.com/identity/protocols/OAuth2ServiceAccount#creatinganaccount
+.. _Authentication Getting Started Guide:
+    https://cloud.google.com/docs/authentication/getting-started
 
 Install Dependencies
 ++++++++++++++++++++