Port existing code to utils.ensure_tree

Now that we have ensure_tree in utils, port most users of
os.makedirs() to use it.

Change-Id: I3256cbb76bd889f99f1e6f3367bd620645f828a1

nova/cloudpipe/pipelib.py

@@ -150,8 +150,7 @@ class CloudPipe(object):
                                                  key_name)
             private_key = result['private_key']
             key_dir = os.path.join(FLAGS.keys_path, context.user_id)
-            if not os.path.exists(key_dir):
-                os.makedirs(key_dir)
+            utils.ensure_tree(key_dir)
             key_path = os.path.join(key_dir, '%s.pem' % key_name)
             with open(key_path, 'w') as f:
                 f.write(private_key)