Try to enable L3 agent extension `fip_qos`

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.

This cherry-pick closes related bug in stable/queens
for linuxbridge scenario job.
In master branch there is no this problem in linuxbridge
scenario job as fip_qos L3 extension is already properly
configured and QoS for Floating IPs works properly.

Partially-Implements blueprint: floating-ip-rate-limit

Related-Bug: #1758033

Change-Id: Ibef48e7842a276fe77c901403d67760871f2b7e0
(cherry picked from commit 6e50a77fc7)

devstack/lib/l3_agent

@@ -0,0 +1,13 @@
+function plugin_agent_add_l3_agent_extension {
+    local l3_agent_extension=$1
+    if [[ -z "$L3_AGENT_EXTENSIONS" ]]; then
+        L3_AGENT_EXTENSIONS=$l3_agent_extension
+    elif [[ ! ,${L3_AGENT_EXTENSIONS}, =~ ,${l3_agent_extension}, ]]; then
+        L3_AGENT_EXTENSIONS+=",$l3_agent_extension"
+    fi
+}
+
+
+function configure_l3_agent {
+    iniset $NEUTRON_L3_CONF agent extensions "$L3_AGENT_EXTENSIONS"
+}

devstack/lib/qos

@@ -18,3 +18,7 @@ function configure_qos {
     configure_qos_core_plugin
     configure_qos_l2_agent
 }
+
+function configure_l3_agent_extension_fip_qos {
+    plugin_agent_add_l3_agent_extension "fip_qos"
+}

devstack/plugin.sh

@@ -4,6 +4,7 @@ source $LIBDIR/dns
 source $LIBDIR/flavors
 source $LIBDIR/l2_agent
 source $LIBDIR/l2_agent_sriovnicswitch
+source $LIBDIR/l3_agent
 source $LIBDIR/ml2
 source $LIBDIR/qos
 source $LIBDIR/ovs
@@ -63,6 +64,12 @@ if [[ "$1" == "stack" ]]; then
                 configure_l2_agent
                 configure_l2_agent_sriovnicswitch
             fi
+            if is_service_enabled q-l3 neutron-l3; then
+                if is_service_enabled q-qos neutron-qos; then
+                    configure_l3_agent_extension_fip_qos
+                fi
+                configure_l3_agent
+            fi
             if [ $NEUTRON_CORE_PLUGIN = ml2 ]; then
                 configure_ml2_extension_drivers
             fi

devstack/settings

@@ -1,4 +1,5 @@
 L2_AGENT_EXTENSIONS=${L2_AGENT_EXTENSIONS:-}
+L3_AGENT_EXTENSIONS=${L3_AGENT_EXTENSIONS:-}
 
 if is_neutron_legacy_enabled; then
     NEUTRON_CORE_PLUGIN=$Q_PLUGIN