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.
[1]https://governance.openstack.org/tc/goals/selected/wallaby/migrate-policy-format-from-json-to-yaml.html
Change-Id: I8b78c7b640ab18ddfc809cb4603decc739d494d1
c5e59b5e9e