Document swift in the host layout section

Adds prose to the host layout section about the swift hosts, and a clear
link to the swift configuration section.

The accompanying graphic was not updated due to missing source files.

Change-Id: I9c109703a9680fae8ce30e43f440c632d1e98fe5
Closes-Bug: #1544898

doc/source/install-guide/configure-swift.rst

@@ -1,5 +1,7 @@
 `Home <index.html>`_ OpenStack-Ansible Installation Guide
 
+.. _configure-swift:
+
 Configuring the Object Storage (swift) service (optional)
 =========================================================
 

doc/source/install-guide/overview-hostlayout.rst

@@ -18,6 +18,10 @@ the use of a sixth host. Block Storage hosts require an LVM volume group named
 requirements" <overview-requirements.html>`_ and `the section
 called "Configuring LVM" <targethosts-configlvm.html>`_ for more information.
 
+If using the optional Object Storage (swift) service, we recommend the use of
+three additional hosts (or some other odd number). See the section
+:ref:`configure-swift` for more information.
+
 The hosts are called target hosts because Ansible deploys the OSA
 environment within these hosts. We recommend a
 deployment host from which Ansible orchestrates the deployment
@@ -57,6 +61,8 @@ services:
 
    -  Dashboard (horizon)
 
+   -  Object storage (swift)
+
 Infrastructure Logging target hosts contain the following services:
 
 -  Rsyslog