Adding an image status transition diagram for dev doc

Now there is no image status transition diagram in the dev
doc, this patch will add one and keep the image source as
a dot file so that it can be updated easily in the future.
See http://www.graphviz.org to get more info about .dot file.

Fixes bug 1244995

Change-Id: I076c461f688be93eab8d3d1fa541990d7b79da5e

doc/source/images_src/image_status_transition.dot

@@ -0,0 +1,47 @@
+/*
+# All Rights Reserved.
+# Copyright 2013 IBM Corp.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+*/
+
+/*
+This file can be compiled by graphviz with issuing the following command:
+
+   dot -Tpng -oimage_status_transition.png image_status_transition.dot
+
+See http://www.graphviz.org to get more info.
+*/
+
+digraph {
+  node [shape="doublecircle" color="#006699" style="filled"
+        fillcolor="#33CCFF" fixedsize="True" width="1.5" height="1.5"];
+
+  "" -> "queued" [label="create image"];
+
+  "queued" -> "active" [label="add location*"];
+  "queued" -> "saving" [label="upload"];
+  "queued" -> "deleted" [label="delete"];
+
+  "saving" -> "active" [label="upload succeed"];
+  "saving" -> "killed" [label="upload fail"];
+  "saving" -> "deleted" [label="delete"];
+
+  "active" -> "queued" [label="remove location*"];
+  "active" -> "pending_delete" [label="delayed delete"];
+  "active" -> "deleted" [label="delete"];
+
+  "killed" -> "deleted" [label="delete"];
+
+  "pending_delete" -> "deleted" [label="after scrub time"];
+}

doc/source/statuses.rst

@@ -55,3 +55,15 @@ Images in Glance can be in one the following statuses:
   This is similiar to `deleted`, however, Glance has not yet removed the
   image data. An image in this state is recoverable.
 
+
+.. figure:: /images/image_status_transition.png
+   :figwidth: 100%
+   :align: center
+   :alt: Image status transition
+
+   This is a representation of how the image move from one status to the next.
+
+   * Add location from zero to more than one.
+
+   * Remove location from one or more to zero by PATCH method which is only
+     supported in v2.