Fix dependency on requests library version

The code was calling property Response.next which is only available in
requests >= 2.15.0 [1]. At present minimum version of requests library
is 2.14.2 in requirements.txt. Instead of bumping its version (impacts
packaging specially to already released OpenStack versions), we can
solve it on our side with this patch.

The error was:
    AttributeError: 'Response' object has no attribute 'next'
    
[1] https://github.com/requests/requests/blob/v2.15.0/HISTORY.rst

Story: 2004641
Task: 28591

Change-Id: I15b496c740b0c7c8970501d08497ba43cbffda2b
diff --git a/octavia_tempest_plugin/tests/test_base.py b/octavia_tempest_plugin/tests/test_base.py
index c70c2f1..e383754 100644
--- a/octavia_tempest_plugin/tests/test_base.py
+++ b/octavia_tempest_plugin/tests/test_base.py
@@ -774,6 +774,8 @@
         :param cookies: Optional cookies to send in the request.
         :param redirect: Is the request a redirect? If true, assume the passed
                          content should be the next URL in the chain.
+        :param timeout: Optional seconds to wait for the server to send data.
+
         :return: boolean success status
 
         :raises: testtools.matchers.MismatchError
@@ -788,6 +790,7 @@
                 self.assertEqual(response_code, req.status_code)
             if redirect:
                 self.assertTrue(req.is_redirect)
-                self.assertEqual(response_content, req.next.url)
+                self.assertEqual(response_content,
+                                 session.get_redirect_target(req))
             elif response_content:
                 self.assertEqual(six.text_type(response_content), req.text)