Implements neutron group based policy resources

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

Implements: blueprint neutron-grouppolicy

Change-Id: Ifa7372c3a1fefadafb74718ea3ff1c48c0d48c4b