Editorial updates to common files, including sentence-style headings and consistency/clarity edits

Partial-Bug: #1250515

backport: havana

Change-Id: I9675dffd130c8aa6343143d9806adb4e0b74a55d
author: diane fleming

doc/common/section_getstart_logical_arch.xml

@@ -26,7 +26,7 @@
     architecture for an OpenStack cloud:</para>
   <!-- Source files in this repository in doc/src/docbkx/common/figures/openstack-arch-havana-v1.zip -->
   <figure xml:id="os-logical-arch">
-    <title>OpenStack logical architecture</title>
+    <title>Logical architecture</title>
     <mediaobject>
       <imageobject>
         <imagedata