Uses open context manager for templated catalogs

Python 3 warns that the file isn't being properly closed. This patch
stops that warning from happening by properly closing the file.

Change-Id: I7dcb08e11dfe4ba679bcde645284b24f8cb126fd
bp: python3

keystone/catalog/backends/templated.py

@@ -100,7 +100,8 @@ class Catalog(core.Driver):
 
     def _load_templates(self, template_file):
         try:
-            self.templates = parse_templates(open(template_file))
+            with open(template_file) as f:
+                self.templates = parse_templates(f)
         except IOError:
             LOG.critical(_LC('Unable to open template file %s'), template_file)
             raise