As per the community goal of migrating the policy file
the format from JSON to YAML[1], we need to do two things:
1. Change the default value of '[oslo_policy] policy_file''
config option from 'policy.json' to 'policy.yaml' with
upgrade checks.
2. Deprecate the JSON formatted policy file on the project side
via warning in doc and releasenotes.
Also replace policy.json to policy.yaml ref from doc and tests.
[1]https://governance.openstack.org/tc/goals/selected/wallaby/migrate-policy-format-from-json-to-yaml.html
Change-Id: I0dbb8484e749e645627756e88ec79c1b26a6414a
fe413fe01d