Document use of kube_tag label

Explain how to select a specific Kubernetes version by specifying the
kube_tag label.

In the process, also fix the broken list of images that must be
mirrored, immediately above kube_tag.

In addition, fix an unrelated whitespace error in
specs/containers-service.rst which would cause tox -e docs to fail.

Change-Id: Ieff1474b74e0b1595c05d945b69bec16bfef9c3b

doc/source/user/index.rst

@@ -336,6 +336,8 @@ the table are linked to more details elsewhere in the user guide.
 +---------------------------------------+--------------------+---------------+
 | `grafana_admin_passwd`_               | (any string)       | "admin"       |
 +---------------------------------------+--------------------+---------------+
+| `kube_tag`_                           | see below          | see below     |
++---------------------------------------+--------------------+---------------+
 | `kube_dashboard_enabled`_             | - true             | true          |
 |                                       | - false            |               |
 +---------------------------------------+--------------------+---------------+
@@ -1087,6 +1089,7 @@ _`container_infra_prefix`
   which assumes an operator has cloned all expected images in
   myregistry.example.com/mycloud.
   Images that must be mirrored:
+
   * docker.io/coredns/coredns:011
   * docker.io/grafana/grafana:latest
   * docker.io/openstackmagnum/kubernetes-apiserver
@@ -1099,6 +1102,13 @@ _`container_infra_prefix`
   * gcr.io/google_containers/kubernetes-dashboard-amd64:v1.5.1
   * gcr.io/google_containers/pause:3.0
 
+_`kube_tag`
+  This label allows users to select `a specific Kubernetes release,
+  based on its container tag
+  <https://hub.docker.com/r/openstackmagnum/kubernetes-apiserver/tags/>`_.
+  If unset, the current Magnum version's default Kubernetes release is
+  installed.
+
 _`kube_dashboard_enabled`
   This label triggers the deployment of the kubernetes dashboard.
   The default value is 1, meaning it will be enabled.

specs/containers-service.rst

@@ -175,7 +175,7 @@ with the Containers Service, and can be controlled only by a Nova virt driver.
  |           +-------+ |  | +-----+                   |
  |                     |  |                           |
  +-----------+---------+  +---------------+-----------+
-             |                            |            
+             |                            |
  +-----------+----+ Compute Host ---------|-----------+
  |                                    +---+---+       |
  |                               +----+ Relay +---+   |