Makedirs only throws oserror, so only catch that

The docs state this only throws OSError so we might
as well just catch only that vs its parent class.

Change-Id: I829996caf7ff256ff4949b8e1e0f27e233bab467

tooz/utils.py

@@ -146,7 +146,7 @@ def ensure_tree(path):
     """
     try:
         os.makedirs(path)
-    except EnvironmentError as e:
+    except OSError as e:
         if e.errno != errno.EEXIST or not os.path.isdir(path):
             raise
         return False