docs: Add magnum guide

Currently just covers deployment of Magnum. Usage information may be
added in future.

Change-Id: I3c1594c73be8e6805f80d51aad2343c084650bc2
(cherry picked from commit 9c8dd72)

Author: markgoddard

doc/source/conf.py

@@ -92,6 +92,7 @@
     'keystone',
     'kolla',
     'kolla-ansible',
+    'magnum',
     'manila',
     'networking-sfc',
     'neutron-vpnaas',

doc/source/reference/containers/index.rst

@@ -9,3 +9,4 @@ including kuryr.
    :maxdepth: 1
 
    kuryr-guide
+   magnum-guide

doc/source/reference/containers/magnum-guide.rst

@@ -0,0 +1,59 @@
+==================================
+Magnum - Container cluster service
+==================================
+
+Magnum is an OpenStack service that provides support for deployment and
+management of container clusters such as Kubernetes. See the
+:magnum-doc:`Magnum documentation </>` for information on using Magnum.
+
+Configuration
+=============
+
+Enable Magnum, in ``globals.yml``:
+
+.. code-block:: yaml
+
+   enable_magnum: true
+
+Optional: enable cluster user trust
+-----------------------------------
+
+This allows the cluster to communicate with OpenStack on behalf of the user
+that created it, and is necessary for the auto-scaler and auto-healer to work.
+Note that this is disabled by default since it exposes the cluster to
+`CVE-2016-7404 <https://nvd.nist.gov/vuln/detail/CVE-2016-7404>`__. Ensure that
+you understand the consequences before enabling this option. In
+``globals.yml``:
+
+.. code-block:: yaml
+
+   enable_cluster_user_trust: true
+
+Optional: private CA
+--------------------
+
+If using TLS with a private CA for OpenStack public APIs, the cluster will need
+to add the CA certificate to its trust store in order to communicate with
+OpenStack. The certificate must be available in the magnum conductor container.
+It is copied to the cluster via user-data, so it is better to include only the
+necessary certificates to avoid exceeding the max Nova API request body size
+(this may be set via ``[oslo_middleware] max_request_body_size`` in
+``nova.conf`` if necessary). In ``/etc/kolla/config/magnum.conf``:
+
+.. code-block:: ini
+
+   [drivers]
+   openstack_ca_file = <path to CA file>
+
+If using Kolla Ansible to :ref:`copy CA certificates into containers
+<admin-tls-ca-in-containers>`, the certificates are located at
+``/etc/pki/ca-trust/source/anchors/kolla-customca-*.crt``.
+
+Deployment
+==========
+
+To deploy magnum and its dashboard in an existing OpenStack cluster:
+
+.. code-block:: console
+
+   kolla-ansible -i <inventory> deploy --tags common,horizon,magnum