Make tempest-multinode-full py2 job on py2

tempest-multinode-full is python2 version
of mutlinode testing for stable branches and
supposed to run on python 2 but it was left during
- I75868d5c9b6630fe78958ff89e58a0aced09a6b3

Change-Id: Ibec72b578424cb278006ac810bdd0359ec23de3f

.zuul.yaml

@@ -339,6 +339,13 @@
     nodeset: openstack-two-node-bionic
     # This job runs on Bionic from stable/stein on.
     branches: ^(?!stable/(ocata|pike|queens|rocky)).*$
+    vars:
+      devstack_localrc:
+        USE_PYTHON3: False
+    group-vars:
+      subnode:
+        devstack_localrc:
+          USE_PYTHON3: False
 
 - job:
     name: tempest-multinode-full