GBP Service Chaining API

This is the first in a series of patches which implements 
Service Chaining in GBP.
This patch introduces the API for Service Chaining.
DB, Plugin and Drivers will be addressed in subsequent patches.

Gerrit Spec: https://review.openstack.org/#/c/125876/
Partially-Implements: blueprint group-based-policy-service-chaining

Change-Id: I2715e50382badccaaa74ad7499db300b127ccbae