Use to_policy_values for policy credentials

The base oslo.context defines to_policy_values with all the information
that it expects a service to require to enforce policy. Use that instead
of throwing everything in to_dict at policy enforcement.

Refer in patch 341905 [1]

[1] https://review.openstack.org/#/c/341905/

Change-Id: I2b964c202de902ee628160c299dca734c8991589
Closes-Bug: #1602081

zun/common/context.py

@@ -88,6 +88,11 @@ class RequestContext(context.RequestContext):
                       'all_tenants': self.all_tenants})
         return value
 
+    def to_policy_values(self):
+        policy = super(RequestContext, self).to_policy_values()
+        policy['is_admin'] = self.is_admin
+        return policy
+
     @classmethod
     def from_dict(cls, values):
         return cls(**values)

zun/common/policy.py

@@ -86,7 +86,7 @@ def enforce(context, rule=None, target=None,
                  expression.
     """
     enforcer = init()
-    credentials = context.to_dict()
+    credentials = context.to_policy_values()
     if not exc:
         exc = exception.PolicyNotAuthorized
     if target is None:
@@ -142,5 +142,5 @@ def check_is_admin(context):
     """
     init()
     target = {}
-    credentials = context.to_dict()
+    credentials = context.to_policy_values()
     return _ENFORCER.enforce('context_is_admin', target, credentials)