docs: Add information on tuning Ansible

Stolen from Kayobe.

Change-Id: I42a2ef84e5fe570e8001908614337e64d035ec99

doc/source/user/ansible-tuning.rst

+==============
+Ansible tuning
+==============
+
+In this section we cover some options for tuning Ansible for performance and
+scale.
+
+SSH pipelining
+--------------
+
+SSH pipelining is disabled in Ansible by default, but is generally safe to
+enable, and provides a reasonable performance improvement.
+
+.. code-block:: ini
+   :caption: ``ansible.cfg``
+
+   [ssh_connection]
+   pipelining = True
+
+Forks
+-----
+
+By default Ansible executes tasks using a fairly conservative 5 process forks.
+This limits the parallelism that allows Ansible to scale. Most Ansible control
+hosts will be able to handle far more forks than this. You will need to
+experiment to find out the CPU, memory and IO limits of your machine.
+
+For example, to increase the number of forks to 20:
+
+.. code-block:: ini
+   :caption: ``ansible.cfg``
+
+   [defaults]
+   forks = 20
+
+Fact caching
+------------
+
+By default, Ansible gathers facts for each host at the beginning of every play,
+unless ``gather_facts`` is set to ``false``. With a large number of hosts this
+can result in a significant amount of time spent gathering facts.
+
+One way to improve this is through Ansible's support for `fact caching
+<https://docs.ansible.com/ansible/latest/user_guide/playbooks_variables.html#caching-facts>`__.
+In order to make this work with Kolla Ansible, it is necessary to change
+Ansible's `gathering
+<https://docs.ansible.com/ansible/latest/reference_appendices/config.html#default-gathering>`__
+configuration option to ``smart``.
+
+Example
+~~~~~~~
+
+In the following example we configure Kolla Ansible to use fact caching using
+the `jsonfile cache plugin
+<https://docs.ansible.com/ansible/latest/plugins/cache/jsonfile.html>`__.
+
+.. code-block:: ini
+   :caption: ``ansible.cfg``
+
+   [defaults]
+   gathering = smart
+   fact_caching = jsonfile
+   fact_caching_connection = /tmp/ansible-facts
+
+You may also wish to set the expiration timeout for the cache via ``[defaults]
+fact_caching_timeout``.

doc/source/user/index.rst

@@ -12,4 +12,5 @@ User Guides
    multi-regions
    operating-kolla
    security
+   ansible-tuning
    troubleshooting