Use python 3 on all nodes in multinode job

In neutron-tempest-plugin-dvr-multinode-scenario we had the same problem
as described in [1] for tempest multinode jobs. It was using python 3
on controller node but python 2 on all subnodes.
This patch changes that to use python3 on all nodes.

[1] https://bugs.launchpad.net/tempest/+bug/1853004

Change-Id: I9ef4daa821342d7462121a8bbde217ff5e0d7e9a
Related-Bug: #1853004
diff --git a/.zuul.yaml b/.zuul.yaml
index 1b82909..325dd58 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -831,6 +831,8 @@
           neutron-trunk: true
           neutron-log: true
           neutron-port-forwarding: true
+        devstack_localrc:
+          USE_PYTHON3: true
         devstack_local_conf:
           post-config:
             $NEUTRON_CONF: