Add GET method to show response with OS-EXT-VIF-NET:net_id param

made a duplicate of the the original get method code,
changed the sample responses to point to the new ones
/api_samples/OS-EXT-VIF-NET/vifs-list-resp.json
/api_samples/OS-EXT-VIF-NET/vifs-list-resp.xml

Added required="true" to <param>, rather than relying on the
default value of "required" attribute. This attribute controls
the appearance of (Optional) text next to params.

Change the plural "interfaces" to singular "interface"
since my understanding is:

You can create a maximum of one virtual interface
per instance per network

According to this page
http://docs.rackspace.com/
servers/api/v2/cn-devguide/content/api_virt_interfaces.html

change all
<doc> tags to
<wadl:doc xml:lang="EN" xmlns="http://docbook.org/ns/docbook">

<p xmlns="http://www.w3.org/1999/xhtml">
to
<para>

use this markup for methods
<wadl:doc xml:lang="EN" xmlns="http://docbook.org/ns/docbook"
title="Show virtual interface"> <para role="shortdesc">

Change-Id: I68b9d4e64a91afb822168fab4df6ce551820c765
Closes-Bug: #1187293