[Docs] Override env.d the new way

We used to have env.d in place, and only need to change it if
necessary.

We now have env.d by default, and override only the relevant part
if necessary.

This should make things clearer in the docs.

Change-Id: Ic425248841baa0645f3395bc6aa010ad81120215
Closes-Bug: 1636018

doc/source/configure-cinder.rst

@@ -218,10 +218,20 @@ Configuring cinder to use LVM
                  iscsi_ip_address: "{{ cinder_storage_address }}"
                limit_container_types: cinder_volume
 
-To use another backend in a
-container instead of bare metal, edit
-the ``/etc/openstack_deploy/env.d/cinder.yml`` and remove the
-``is_metal: true`` stanza under the ``cinder_volumes_container`` properties.
+To use another backend in a container instead of bare metal,
+copy the ``env.d/cinder.yml`` to
+``/etc/openstack_deploy/env.d/cinder.yml`` file and change the
+``is_metal: true`` stanza under the ``cinder_volumes_container`` properties
+to ``is_metal: false``.
+
+Alternatively, you can also selectively override, like this:
+
+.. code-block:: yaml
+
+   container_skel:
+     cinder_volumes_container:
+       properties:
+         is_metal: false
 
 Configuring cinder to use Ceph
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -231,7 +241,19 @@ the API and backend. When using any forms of network storage
 (iSCSI, NFS, Ceph) for cinder, the API containers can be considered
 as backend servers. A separate storage host is not required.
 
-In ``env.d/cinder.yml`` remove ``is_metal: true``
+Copy the ``env.d/cinder.yml`` to
+``/etc/openstack_deploy/env.d/cinder.yml`` file and change the
+``is_metal: true`` stanza under the ``cinder_volumes_container`` properties
+to ``is_metal: false``.
+
+Alternatively, you can also selectively override, like this:
+
+.. code-block:: yaml
+
+   container_skel:
+     cinder_volumes_container:
+       properties:
+         is_metal: false
 
 #. List of target hosts on which to deploy the cinder API. We recommend
    that a minimum of three target hosts are used for this service.