Pin neutron-tempest-plugin to v1.6.0

Later versions of the neutron tempest plugin depend on a
more recent version of tempest than is used by the OSA
wallaby release and earlier stable releases. Later versions
of this plugin require the
'qos_limit_bandwidth_rules_client' component.

Change-Id: Ifba6836d824e7e6b956a81271d95d6dabe6c4163

vars/main.yml

@@ -119,7 +119,7 @@ _tempest_plugins:
   neutron:
     - name: neutron-tempest-plugin
       repo: https://opendev.org/openstack/neutron-tempest-plugin
-      branch: master
+      branch: 1.6.0
       install: "{{ tempest_service_available_neutron | bool }}"
   novajoin:
     - name: novajoin-tempest-plugin