Use the public keystone interface

This should not have any consequences on the functionality but avoids
having to deal with endpoints that are usually not accessible from the
outside world (the admin one). The use of keystone is mostly to get
 the user list so no real admin operations are done.

caso/keystone_client.py

@@ -41,4 +41,4 @@ def get_session(conf, project):
 def get_client(conf, project):
     """Return a client for Keystone."""
     sess = get_session(conf, project)
-    return ks_client_v3.Client(session=sess)
+    return ks_client_v3.Client(session=sess, interface='public')