Use neutron-legacy on subnodes in devstack zuul job

There are still some issues with lib/neutron thus neutron-legacy is used
on controller node in multinode jobs and in single node jobs.
But in "group-vars" in devstack job it was configured to use
lib/neutron which can cause some problems in multinode jobs.

So lets switch to neutron-legacy on subnodes also until lib/neutron
will be ready to use everywhere.

Closes-Bug: #1834079

Change-Id: I0d7f9f2baaee2836a719f199939156bd4f53f778
(cherry picked from commit 1b15176b05)

.zuul.yaml

@@ -460,7 +460,10 @@
           n-cpu: true
           placement-client: true
           # Neutron services
-          neutron-agent: true
+          # We need to keep using the neutron-legacy based services for
+          # now until all issues with the new lib/neutron code are solved
+          q-agt: true
+          # neutron-agent: true
           # Cinder services
           c-bak: true
           c-vol: true