Add Overview subsection to Proposed Change section

In Juno, I found I had quite a bit of detail under "Proposed Change". It
would have been useful to be able to subdivide those details into
sections. However, due to the way the template and unit tests were
structured, both first and second level sections were validated against
the expected structure.

This change adds a second-level section under Proposed Change called
Overview. The tests do not care about third-level subsections, so this
new section can be further broken up as necessary to keep the spec
readable.

Change-Id: I7b840329567a0c1bb3154dd344fcf9ddc9c9dbdc

specs/kilo-template.rst

@@ -49,6 +49,9 @@ A detailed description of the problem:
 Proposed Change
 ===============
 
+Overview
+--------
+
 Here is where you cover the change you propose to make in detail. How do you
 propose to solve this problem?
 

specs/kilo/cinder_ha.rst

@@ -27,6 +27,9 @@ fail, volumes hosted by that node would be lost forever.
 Proposed Change
 ===============
 
+Overview
+--------
+
 We aim at introducing support for the configuration of Cinder's Ceph backend
 driver and for the deployment of a Ceph storage for use with Cinder.
 

specs/kilo/tripleo-enable-dvr.rst

@@ -32,6 +32,9 @@ additional settings.
 Proposed Change
 ===============
 
+Overview
+--------
+
 In the tripleo-image-elements, move the current ``neutron-network-node`` element
 to an element named ``neutron-router``, which will be responsible for doing the
 installation and configuration work required to install the ``neutron-l3-agent``

specs/kilo/tripleo-review-standards.rst

@@ -42,6 +42,10 @@ is in favor of it.
 
 Proposed Change
 ===============
+
+Overview
+--------
+
 When appropriate, allow a core reviewer to approve a change even if the
 latest patch set does not have 2 +2's.  Specifically, this should be used
 under the following circumstances: