38bf2470b8
Prior to this change zuul-sphinx would error if there were directories in zuul.d that needed to be processed recursively. This is because it would try to open the directory as a normal file. Fix this by using os.walk instead of os.listdir to iterate through the contents of the zuul.d directory. Additionally filter for .yaml files only as Zuul does this as well. Change-Id: Id2d828d7842a86b7f89a2ce28a084e70f91d3f55