Represent External Connectivity in GBP: API-DB

Today, the GBP model only represents east-west traffic policies.
This new API and reference implementation
allows north-south traffic in a GBP enabled cloud
partial implements blueprint external-connectivity

Change-Id: I9799e6e8ce439bf45d5aef3d800c90df29c06fb3
Merge "Represent External Connectivity in GBP: API-DB"