Traffic Steering as a Service Chaining provider

The Service Chaining API provided by the Group-based Policy (GBP) project currently allows the instantiation of chains of network services based on the deployment requirements of applications. However, it is currently only possible to use either Firewall or Load Balancer services based on Heat orchestration templates, and chain them together. This blueprint proposes a set of modifications to allow a new flavor of service functions to be deployed using Group-based Policy. This flavor is called Traffic Steering (TS) and is particularly helpful in chaining services that are not known to Neutron and only manifested as Neutron ports (e.g. a user managed service VM). Furthermore, these virtual machines can be used as service functions, giving cloud tenants or NFV operators the freedom to choose their own network services based on VMs or anything else that can be mapped to Neutron ports, and chain them together. This spec builds upon the spec presented in https://review.openstack.org/#/c/92477/ and its underlying implementation, but now adapted and aimed at Group-based Policy.