Added ML2 controller node section to Neutron chapter

As part of the installation guide improvement project, I performed
the following operations on the Neutron chapter:

1) Modified structure to retain existing OVS plug-in sections while
   adding ML2 plug-in sections.
2) Added ML2 plug-in controller node section.

This patch primarily covers content. Future patches may address
structure and/or appearance issues.

Change-Id: Ic6955316f51b3b3ada45e27e7a6d48bf6a815004
Partial-Bug: #1291071
Implements: blueprint networking-install-guide-improvements

doc/glossary/glossary-terms.xml

@@ -2093,6 +2093,14 @@
                     passes network traffic between different networks.</para>
             </glossdef>
         </glossentry>
+        <glossentry>
+            <glossterm>Generic Routing Encapsulation (GRE)</glossterm>
+            <glossdef>
+                <para>Protocol that encapsulates a wide variety of network
+                    layer protocols inside virtual point-to-point links.
+                </para>
+            </glossdef>
+        </glossentry>
         <glossentry>
             <glossterm>glance</glossterm>
             <glossdef>

doc/install-guide/ch_networking.xml

@@ -32,9 +32,24 @@
   <section xml:id="section_neutron-networking">
     <title>Networking (neutron)</title>
     <xi:include href="section_neutron-concepts.xml"/>
-    <xi:include href="section_neutron-controller-node.xml"/>
-    <xi:include href="section_neutron-network-node.xml"/>
-    <xi:include href="section_neutron-compute-node.xml"/>
+    <section xml:id="section_neutron-networking-ovs">
+    <title>Open vSwitch (OVS) plug-in</title>
+      <note>
+        <para>We recommend using the Open vSwitch (OVS) plug-in until we
+          complete documentation for the Modular Layer 2 (ML2) plug-in.</para>
+      </note>
+      <xi:include href="section_neutron-controller-node.xml"/>
+      <xi:include href="section_neutron-network-node.xml"/>
+      <xi:include href="section_neutron-compute-node.xml"/>
+    </section>
+    <section xml:id="section_neutron-networking-ml2">
+    <title>Modular Layer 2 (ML2) plug-in</title>
+      <warning>
+        <para>We are adding this documentation for Icehouse. Until complete,
+          we recommend using the Open vSwitch (OVS) plug-in.</para>
+      </warning>
+      <xi:include href="section_neutron-ml2-controller-node.xml"/>
+    </section>
     <xi:include href="section_neutron-initial-networks.xml"/>
   </section>
   <section xml:id="section_nova-networking">