Revive legacy job

The job against newton hasn't been running for five years, switch to run
against some oldish, but still supported stable branch.

Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I89b38cb67deed0a2cdd7ce62c1e3e068de59cec1

.zuul.yaml

@@ -101,16 +101,9 @@
     parent: openstacksdk-functional-devstack-base
     description: |
       Run openstacksdk functional tests against a legacy devstack
+    nodeset: openstack-single-node-bionic
     voting: false
-    vars:
-      devstack_localrc:
-        ENABLE_IDENTITY_V2: true
-        FLAT_INTERFACE: br_flat
-        PUBLIC_INTERFACE: br_pub
-      tox_environment:
-        OPENSTACKSDK_USE_KEYSTONE_V2: 1
-        OPENSTACKSDK_HAS_NEUTRON: 0
-    override-branch: stable/newton
+    override-branch: stable/ussuri
 
 - job:
     name: openstacksdk-functional-devstack
@@ -471,6 +464,8 @@
             voting: false
         - openstacksdk-functional-devstack-ironic:
             voting: false
+        - openstacksdk-functional-devstack-legacy:
+            voting: false
         - osc-functional-devstack-tips:
             voting: false
         # Ironic jobs, non-voting to avoid tight coupling