Magnum API previously passed magnum.objects.cluster.Cluster objects as
the target argument to magnum.common.policy.enforce(). However, enforce()
expects target to be a mutable mapping, as it adds an entry for
trustee_domain_id which is used by the magnum policy.json. This causes
cluster detailed GET requests to fail with the following message:
AttributeError: 'Cluster' object has no attribute 'trustee_domain_id'
This change uses the as_dict() method of the magnum RPC objects to
provide a mutable mapping to the policy enforcer.
Change-Id: I54b136243afff9e0fadae3be4b36cad1679e5721
Closes-Bug: #1689797
f1326626b9