GBP Service Chaining DB

This patch implements DB model for Service Chaining.
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: Ibd41779c98530eb54f45b25f645c4b1dc46fec9d