Default value of 'CONF.oslo_policy.policy_file' config option
has been changed from 'policy.json' to 'policy.yaml'. If new default
file 'policy.yaml' does not exist but old default 'policy.json' exist
then fallback to use old default file.
An upgrade checks is added to check the policy_file format and
fail upgrade checks if it is JSON formatted.
Added a warning in policy doc about JSON formatted file is deprecated,
also removed all the reference to policy.json file in doc as well as
in tests.
Related Blueprint: policy-json-to-yaml
Closes-Bug: #1875418
Change-Id: Ic4d3b998bb9701cb1e3ef12d9bb6f4d91cc19c18
fe545dbe5f