FWaaS Insertion Model on Routers

Specification to change the FWaaS insertion model to associate a Firewall
with specified Routers as opposed to the current model of association
with all Routers in the tenant.

APIImpact

Change-Id: I14085af97f3bbad4530a3ff97c107b6dbbc8bd1d
Implements: blueprint fwaas-router-insertion
Merge "FWaaS Insertion Model on Routers"