Fix ReleaseNotes build

for some reason (clearly another change somewhere in the Sphinx)
extlinks extension used in the releasenotes is failing when extlink
caption is not having substitues same as in the link itself.

>     title = caption % part
>             ~~~~~~~~^~~~~~
> TypeError: not all arguments converted during string formatting

Now the extension supports setting link capturion to None which works as
we need.

Change-Id: Ie1a61e9b3ee5d727d59525efe34aea696538e2b3

Author: gtema

releasenotes/source/conf.py

@@ -63,15 +63,15 @@
 extlinks = {
     'lpbug': (
         'https://bugs.launchpad.net/bugs/%s',
-        'Bug ',
+        'Bug %s',
     ),
     'oscbp': (
         'https://blueprints.launchpad.net/python-openstackclient/+spec/%s',
-        '',
+        None,
     ),
     'oscdoc': (
         'https://docs.openstack.org/python-openstackclient/latest/%s.html',
-        '',
+        None,
     ),
 }