Horizon panel, tabs, and tables for policies

contrib/devstack/lib/
* congress: Copy files needed for Horizon integration.

contrib/horizon/
* _50_policies.py: Configuration for plugin panel.
* congress.py: Wrapper around Congress Python client.
* policies/*: New panel, tabs, and tables for displaying policy details, policy
  rules, and policy tables.

Implements: blueprint horizon-read-policies
Change-Id: I86e75f5fd7db79d8f98b86afb4d6447b298d41e4
Merge "Horizon panel, tabs, and tables for policies"