This commit allows horizon to handle deprecated policy rules.
The approach is explained in the document updated by this change.
oslo.policy requirement is updated. oslo.policy 3.2.0 is chosen
just because it is the first release in Victoria cycle.
requirements.txt and lower-constraints.txt are updated accordingly
including oslo.policy dependencies.
Change-Id: If5059d03f6bd7e94796065aa1b51c0c23ac85f5e