Adds devstack configs to try to enable the L3 agent
extension `fip_qos` by checking the API service
extension router, agent and QoS.
Once this L3 agent extension works during the devstack
zuul job installation, we can continue to run the
neutron-tempest-plugin tests for floating IP QoS.
Partially-Implements blueprint: floating-ip-rate-limit
Change-Id: Ibef48e7842a276fe77c901403d67760871f2b7e0