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

(Patch series identifier: GPM-PLG-1)

This patch changes the plugin layer for the initial set of Group
Policy API resources to use the Group Policy Mapping DB layer and its
unit tests. Subsequent patches will add drivers rendering policy
constructs via the mapped resources.

Author:            Bob Kukura <kukura@noironetworks.com>
Co-Authored-By:    Sumit Naiksatam <sumitnaiksatam@gmail.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>

Change-Id: Ic277a248a1341393623db8bd5910a35e63657095