Group policy API-1 HEAT resources: EP, EPG, L2, L3

This is the first in a series of patches which implement
resources for neutron group policy APIs.

This patch implements four new resources:
    Endpoint
    EndpointGroup
    L2-Policy
    L3-Policy

Change-Id: Iac413aaa460bffa5c6f111e2e16beb9507a3d24a
Implements: blueprint group-based-policy-automation