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

(Patch series identifier: GP-PLG-1)
This patch includes the plugin and the policy driver framework to realize the
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.
Gerrit Spec: https://review.openstack.org/#/c/89469
Partially-implements: blueprint group-based-policy-abstraction
Change-Id: Iffaf41d84e3acab6cf628ac0305489e4177c05de
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>
Merge "Group Policy Plugin-1: EP, EPG, L2 Policy, L3 Policy"