Add ironic hypervisor doc

Nova documentation was missing information documentation about
Ironic virt driver.

Change-Id: I2431dc3db94da44001eefdead50607eab662d16f
Closes-Bug: #1852446

doc/source/admin/configuration/hypervisor-ironic.rst

@@ -0,0 +1,59 @@
+Ironic
+======
+
+Introduction
+------------
+The ironic hypervisor driver wraps the Bare Metal (ironic) API,
+enabling Nova to provision baremetal resources using the same
+user-facing API as for server management.
+
+This is the only driver in nova where one compute service can map to many hosts
+, meaning a ``nova-compute`` service can manage multiple ``ComputeNodes``. An
+ironic driver managed compute service uses the ironic ``node uuid`` for the
+compute node ``hypervisor_hostname`` (nodename) and ``uuid`` fields.
+The relationship of ``instance:compute node:ironic node`` is ``1:1:1``.
+
+Scheduling of bare metal nodes is based on custom resource classes, specified
+via the ``resource_class`` property on a node and a corresponding resource
+property on a flavor (see the `flavor documentation`_).
+The RAM and CPU settings on a flavor are ignored, and the disk is only used to
+determine the root partition size when a partition image is used (see the
+`image documentation`_).
+
+
+.. _flavor documentation: https://docs.openstack.org/ironic/latest/install/configure-nova-flavors.html
+.. _image documentation: https://docs.openstack.org/ironic/latest/install/configure-glance-images.html
+
+Configuration
+-------------
+
+- `Configure the Compute service to use the Bare Metal service
+  <https://docs.openstack.org/ironic/latest/install/configure-compute.html>`_.
+
+- `Create flavors for use with the Bare Metal service
+  <https://docs.openstack.org/ironic/latest/install/configure-nova-flavors.html>`__.
+
+- `Conductors Groups
+  <https://docs.openstack.org/ironic/latest/admin/conductor-groups.html>`_.
+
+
+Scaling and Performance Issues
+------------------------------
+
+- The ``update_available_resource`` periodic task reports all the resources
+  managed by Ironic. Depending the number of nodes, it can take a lot of time.
+  The nova-compute will not perform any other operations when this task is
+  running. You can use conductor groups to help scale, by setting
+  :oslo.config:option:`ironic.partition_key`.
+
+
+
+Known limitations / Missing features
+------------------------------------
+
+* Migrate
+* Resize
+* Snapshot
+* Pause
+* Shelve
+* Evacuate

doc/source/admin/configuration/hypervisors.rst

@@ -19,6 +19,7 @@ Hypervisors
    hypervisor-virtuozzo
    hypervisor-powervm
    hypervisor-zvm
+   hypervisor-ironic
 
 OpenStack Compute supports many hypervisors, which might make it difficult for
 you to choose one. Most installations use only one hypervisor.  However, you