Ensure test for Neutron GET / uses right URL

Previously, the logic in the API test for Neutron GET / did not always
determine the form of the unversioned endpoint correctly. For example,
when running Neutron on a shared port instead of on its own service
port, e.g. http://host/network/v2.0 as the endpoint, the test would
incorrectly run against http://host/ instead of http://host/network.

Now, ensure that the correct form of the unversioned endpoint is always
used in the test.

Change-Id: I9dcd30fe6669bdf570149ed317b16005c21f6291
Closes-Bug: #1703959
diff --git a/tempest/lib/services/network/versions_client.py b/tempest/lib/services/network/versions_client.py
index a9c3bbf..f87fe87 100644
--- a/tempest/lib/services/network/versions_client.py
+++ b/tempest/lib/services/network/versions_client.py
@@ -15,7 +15,6 @@
 import time
 
 from oslo_serialization import jsonutils as json
-from six.moves import urllib
 
 from tempest.lib.services.network import base
 
@@ -25,9 +24,7 @@
     def list_versions(self):
         """Do a GET / to fetch available API version information."""
 
-        endpoint = self.base_url
-        url = urllib.parse.urlparse(endpoint)
-        version_url = '%s://%s/' % (url.scheme, url.netloc)
+        version_url = self._get_base_version_url()
 
         # Note: we do a raw_request here because we want to use
         # an unversioned URL, not "v2/$project_id/".