Do not use yaml.load(file)

Now a Loader is required, as the default loader is considered unsafe.
See [1] for details.

[1] https://msg.pyyaml.org/load

Change-Id: I08af1afd6df097fab33701db89c656ebfe981b30

dragonflow/controller/datapath_layout.py

@@ -53,8 +53,9 @@ def get_datapath_layout(path=None):
     if path is None:
         path = cfg.CONF.df.datapath_layout_path
 
+    # See https://msg.pyyaml.org/load
     with open(path) as f:
-        raw_layout = yaml.load(f)
+        raw_layout = yaml.safe_load(f)
 
     vertices = tuple(
         Vertex(