Fix api_version filter for KeystoneV3AuthProvider

This add a missing '/' character during the replacement of api version.
Related to bp unit-tests

Change-Id: I83c2256c38ef91c9f36a564375a87f23bc8e0e25
diff --git a/tempest/auth.py b/tempest/auth.py
index e1ba13b..8cb3b2c 100644
--- a/tempest/auth.py
+++ b/tempest/auth.py
@@ -382,7 +382,7 @@
             path = "/" + filters['api_version']
             noversion_path = "/".join(parts.path.split("/")[2:])
             if noversion_path != "":
-                path += noversion_path
+                path += "/" + noversion_path
             _base_url = _base_url.replace(parts.path, path)
         if filters.get('skip_path', None) is not None:
             _base_url = _base_url.replace(parts.path, "/")