Adds NSX integration overview

Adds overview text to network architecture section. Includes a
network diagram demonstrating the interaction of the the
neutron and compute nodes with the NSX controller.

Change-Id: I0b5088cee8887b21c60c931e98bb4ec28f40cced
Closes-Bug: 1315968

doc/admin-guide-cloud/networking/section_networking_arch.xml

@@ -263,4 +263,29 @@
         tenants as part of the network creation process.</para>
     </formalpara>
 </section>
+    <section xml:id="NSX_overview">
+        <title>VMware NSX integration</title>
+        <para>OpenStack Networking uses the NSX plugin for Networking to
+        integrate with an existing VMware vCenter deployment. When installed on
+        the network nodes, the NSX plugin enables a NSX controller to centrally
+        manage configuration settings and push them to managed network nodes.
+        Network nodes are considered managed when they're added as hypervisors
+        to the NSX controller.</para>
+        <para>The diagram below depicts an example NSX deployment and
+        illustrates the route inter-VM traffic takes between separate Compute
+        nodes. Note the placement of the VMware NSX plugin and the
+        <systemitem class="service">neutron-server</systemitem> service on the
+        network node. The NSX controller features centrally with a green line to
+        the network node to indicate the management relationship:</para>
+        <figure>
+            <title>VMware NSX overview</title>
+            <mediaobject>
+                <imageobject>
+                    <imagedata
+                        fileref="../../common/figures/vmware_nsx.png"
+                        format="PNG" contentwidth="6in"/>
+                </imageobject>
+            </mediaobject>
+        </figure>
+    </section>
 </section>