Improved installation guide diagrams

I improved the neutron initial networks diagram in the
installation guide.

Change-Id: I52593d97fbbfe1f8439cbf68a5fdc7f5d19364f8
Implements: blueprint installation-guide-improvements
backport: juno

doc/install-guide/section_neutron-initial-networks.xml

@@ -15,13 +15,16 @@
     creating this infrastructure, we recommend that you
     <link linkend="neutron_initial-networks-verify">verify
       connectivity</link> and resolve any issues before proceeding further.
-  </para>
+    <xref linkend="neutron_figure-neutron-initial-networks"/> provides a
+    basic architectural overview of the components that Networking implements
+    for the initial networks and shows how network traffic flows from the
+    instance to the external network or Internet.</para>
   <figure xml:id="neutron_figure-neutron-initial-networks">
     <title>Initial networks</title>
     <mediaobject>
       <imageobject>
-        <imagedata scale="50"
-          fileref="figures/installguide_neutron-initial-networks.png"/>
+        <imagedata contentwidth="6in"
+          fileref="../common/figures/installguide-neutron-initialnetworks.png"/>
       </imageobject>
     </mediaobject>
   </figure>