Group Policy API-3 HEAT resources: Contracts

This is the third patch in the Group Policy resources implementation
series. This patch implements:
    Contracts

In the context of larger Group Policy model, Contracts contain Policy
Rules and Endpoint Groups consume or provide Contracts.

Change-Id: I9d92913f656af5a5d177f0cff5f3b306ced06fdf
Implements: blueprint group-based-policy-automation