Group Policy DB-1: EP, EPG, L2 Policy, L3 Policy

(Patch series identifier: GP-DB-1)
This patch includes the DB layer for for Endpoint, Endpoint Group, L2
Policy, and L3 Policy resources.
In the context of the larger Group Policy Model, the Endpoint Group resource
references Contracts, which will be introduced in a subsequent patch.
The Plugin layer to back these resources will also be introduced in
subsequent patches.
Gerrit Spec: https://review.openstack.org/#/c/89469
Partially-implements: blueprint group-based-policy-abstraction
Change-Id: I6e1843e7f765ce004129a5a062b67eb57f3f26f6
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>
Merge "Group Policy DB-1: EP, EPG, L2 Policy, L3 Policy"