Create an admin context to refresh template files

Its impractical to pass in the current context to
TemplateFiles._refresh, and it will soon be mandatory for all db api
context arguments to be not None. This change creates a short-lived
context just for the refresh.

It will have the same effect as the current behaviour of creating a
new session, since the a new session will be associated with the
context, so there is no danger of returning stale data from the
current session's identity map.

This change also makes the context argument mandatory for any raw
templates operations that might need one.

Change-Id: I156096d736aac4999a3eebe077ed50ef7384ca02

heat/engine/template.py

@@ -133,7 +133,7 @@ class Template(collections.Mapping):
         return cls(t.template, template_id=template_id, env=env,
                    files=t_files)
 
-    def store(self, context=None):
+    def store(self, context):
         """Store the Template in the database and return its ID."""
         rt = {
             'template': self.t,

heat/engine/template_files.py

@@ -15,6 +15,7 @@ import collections
 import six
 import weakref
 
+from heat.common import context
 from heat.common.i18n import _
 from heat.db import api as db_api
 from heat.objects import raw_template_files
@@ -89,12 +90,13 @@ class TemplateFiles(collections.Mapping):
         self._refresh()
 
     def _refresh(self):
-        rtf_obj = db_api.raw_template_files_get(None, self.files_id)
+        ctxt = context.get_admin_context()
+        rtf_obj = db_api.raw_template_files_get(ctxt, self.files_id)
         _files_dict = ReadOnlyDict(rtf_obj.files)
         self.files = _files_dict
         _d[self.files_id] = _files_dict
 
-    def store(self, ctxt=None):
+    def store(self, ctxt):
         if not self.files or self.files_id is not None:
             # Do not to persist an empty raw_template_files obj. If we
             # already have a not null self.files_id, the (immutable)