Add a sleep for the service account to propagate. (GoogleCloudPlatform#6895)

I suspect that sometimes this method fails
because there is a race condition when creating the new
servie account. Adding a service account is not a LRO,
but it's also no atomic. The information about the new
account needs a moment to propagate through the system.

Closes GoogleCloudPlatform#6894

## Description

Fixes #<ISSUE-NUMBER>

Note: It's a good idea to open an issue first for discussion.

## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [ ] These samples need a new **API enabled** in testing projects to pass (let us know which ones)
- [ ] These samples need a new/updated **env vars** in testing projects set to pass (let us know which ones)
- [x] Please **merge** this PR for me once it is approved.
- [ ] This sample adds a new sample directory, and I updated the [CODEOWNERS file](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/.github/CODEOWNERS) with the codeowners for this sample

Author: m-strzelczyk

compute/oslogin/service_account_ssh_test.py

@@ -1,9 +1,10 @@
-# Copyright 2019, Google, Inc.
+# Copyright 2019 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
+#     https://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,
@@ -112,6 +113,10 @@ def setup_resources(
             'accountId': test_id
         }).execute()
 
+    # Wait for the creation to propagate through the system, so the
+    # following calls don't fail sometimes.
+    time.sleep(5)
+
     # Grant the service account access to itself.
     iam.projects().serviceAccounts().setIamPolicy(
         resource='projects/' + project + '/serviceAccounts/' + account_email,