Use to_policy_values for enforcing policy

oslo_context's to_policy_values provides a standard list of parameters
that policy should be able to be enforced upon. The combination of this
and from_environ lets oslo.context handle adding new values to policy
enforcement.

Closes-Bug: #1602081
Change-Id: I8f70580e7209412800aa7b948602b003392ef238

cinder/context.py

@@ -145,6 +145,13 @@ class RequestContext(context.RequestContext):
                    user_domain=values.get('user_domain'),
                    project_domain=values.get('project_domain'))
 
+    def to_policy_values(self):
+        policy = super(RequestContext, self).to_policy_values()
+
+        policy['is_admin'] = self.is_admin
+
+        return policy
+
     def elevated(self, read_deleted=None, overwrite=False):
         """Return a version of this context with admin flag set."""
         context = self.deepcopy()