Added docs and example to work with identity v2.0

Author: haphan

doc/services/identity/v2/authentication.rst

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+Authentication
+==============
+
+Identity v2.0
+-------------
+
+We recommend users to use identity v3 when possible.
+
+RackSpace cloud and OVH cloud, however, are still using Keystone v2.0. This following code will work for such providers.
+
+
+.. sample:: identity/v2/authentication.php

doc/services/identity/v2/index.rst

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+Identity v2
+===========
+
+.. toctree::
+:maxdepth: 3
+
+      authentication

samples/identity/v2/authentication.php

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+<?php
+
+require 'vendor/autoload.php';
+
+use GuzzleHttp\Client;
+use GuzzleHttp\HandlerStack;
+use OpenStack\Common\Transport\Utils as TransportUtils;
+use OpenStack\OpenStack;
+
+$authUrl = 'https://keystone.example.com:5000/v2.0';
+
+$options = [
+    'authUrl'         => $authUrl,
+    'region'          => 'RegionOne',
+    'username'        => 'foo',
+    'password'        => 'bar',
+    'tenantName'      => 'baz',
+    'identityService' => new Client(
+        [
+            'base_uri' => TransportUtils::normalizeUrl($authUrl),
+            'handler'  => HandlerStack::create(),
+        ]
+    ),
+];
+
+/** @var OpenStack $openstack */
+$openstack = new OpenStack($options);
+
+/** @var \OpenStack\Compute\v2\Models\Server[] $servers */
+$servers = $openstack->computeV2()->listServers();
+
+