Deprecate "extension:xxx" policies but preserve bw compatibility

Blueprint make-authz-orthogonal

Patch #4 for this blueprint.
With this change, if policies starting with 'extension' are still
present in policy.json, a deprecation warning will be emitted, and
the policy will be translated in the corresponding, new-style,
policy(ies).

Change-Id: Iccf0f36cc2547c35c66854b8a3fdb7185b5214bd
Merge "Deprecate "extension:xxx" policies but preserve bw compatibility"