Add good use cases for manually running config-download playbooks.

This was created based on the feedback from the community.
Real world use cases are explained for config-download and
also what advantages it brings.

Change-Id: I995167f00df5ccd9dfa2fd633714eab9769ebfd4
Signed-off-by: Luke Short <ekultails@gmail.com>

deploy-guide/source/deployment/ansible_config_download_differences.rst

@@ -106,3 +106,28 @@ from the steps to enable the tripleo-admin user via ssh are shown.
 .. include:: deployment_status.rst
 
 .. include:: deployment_log.rst
+
+config-download Use Cases
+-------------------------
+config-download exposes the ability to manually run the ``ansible-playbook``
+command against the playbooks that are generated for the deployment. This leads
+to many advantages over the older Heat deployment model.
+
+-  Test deployments. Using the
+   ``ansible-playbook --check --diff deploy_steps_playbook.yaml``
+   arguments will not modify an existing deployment. Instead, it will only show
+   any changes that would be made.
+-  Development environment testing. Ansible variables can be modified to do
+   quick testing. Once verified, Heat environment templates need to be updated
+   to reflect the change permanently. Then the config-download content should
+   be re-generated by running ``openstack overcloud deploy --stack-only``.
+-  Run specific tasks. It is possible to run certain parts of a deployment by
+   using ``--tags``.
+-  Prepare the deployment or update ahead of time and then run the playbooks
+   later. The operations around a deployment can be done at different times to
+   minimize risk.
+-  Integration with CI/CD. Additional checks and verification can be added to
+   a CI/CD pipeline relating to updating Heat templates and the Ansible
+   config-download content.
+-  AWX or Ansible Tower integration. Ansible content can be imported and ran
+   through a scalable and distributed system.