wsgi: Don't create, use lock in same line

As noted on the mailing list some time back [1], pylint flags this as a
useless lock [2]. Make it non-useless.

[1] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/message/CZVC6SEMUSEH7UT5LDHOWL7WBZ2OXUWZ/
[2] https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/useless-with-lock.html

Change-Id: Ia0d3899d84db5fce92fad17d19f375681debe864
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>

keystone/wsgi/api.py

@@ -17,6 +17,7 @@ import threading
 from keystone.server import wsgi
 
 application = None
-with threading.Lock():
+lock = threading.Lock()
+with lock:
     if application is None:
         application = wsgi.initialize_public_application()