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 code.
[1]https://governance.openstack.org/tc/goals/selected/wallaby/migrate-policy-format-from-json-to-yaml.html
Change-Id: I1aa12bcd2638390f25d57ce8abeeec248121dc02
157f358057