Group Policy DB-3: Contracts

(Patch series identifier: GP-DB-3)

This patch includes the DB layer for the Contracts resource.

In the context of the larger Group Policy Model, Contracts contain Policy
Rules and Endpoint Groups consume or provide Contracts. Policy Rule were
introduces in GP-API-2 patch, and Endpoint Groups were introduced in GP-API-1
patch (both are earlier patches in this series).

The Plugin layer to back the Contract resource  will also be introduced in a
subsequent patchs.

Partially-implements: blueprint group-based-policy-abstraction

Change-Id: I0a3199faf3003fdff4bd65ba9da5fb6257bce7b8
Author: Sumit Naiksatam <sumitnaiksatam@gmail.com>
Co-Authored-By: Bob Kukura <kukura@noironetworks.com>
Co-Authored-By: Stephen Wong <s3wong@midokura.com>
Co-Authored-By: Mohammad Banikazemi <mb@us.ibm.com>
Co-Authored-By: Mandeep Dhami <dhami@noironetworks.com>
Co-Authored-By: Ivar Lazzaro <ivarlazzaro@gmail.com>