Decode additional heat files

All file contents should be json serializable

Change-Id: I7da20f1f68ecd73b83bed237d09314eaffe6d4f6
Closes-Bug: https://storyboard.openstack.org/#!/story/2002002

openstack/cloud/_heat/utils.py

@@ -46,7 +46,7 @@ def read_url_content(url):
 
     if content:
         try:
-            content.decode('utf-8')
+            content = content.decode('utf-8')
         except ValueError:
             content = base64.encodestring(content)
     return content