Syncing changes from oslo-incubator policy engine

In this patch, I have copied required changes from oslo-incubator
policy module from commit 0da5de6b548d34300a75f80bf87d6a809c609d2f.

This changes are required for implementation of blueprint
restrict-downloading-images-protected-properties.

blueprint: restrict-downloading-images-protected-properties
Change-Id: I00653411cd0a92503a3096c0dde07e1d6a229353

glance/openstack/common/policy.py

@@ -57,6 +57,7 @@ as it allows particular rules to be explicitly disabled.
 """
 
 import abc
+import ast
 import re
 import urllib
 
@@ -775,6 +776,13 @@ class GenericCheck(Check):
 
         # TODO(termie): do dict inspection via dot syntax
         match = self.match % target
-        if self.kind in creds:
-            return match == six.text_type(creds[self.kind])
-        return False
+
+        try:
+            # Try to interpret self.kind as a literal
+            leftval = ast.literal_eval(self.kind)
+        except ValueError:
+            try:
+                leftval = creds[self.kind]
+            except KeyError:
+                return False
+        return match == six.text_type(leftval)