Move stable branch gate job to stable/ussuri

We have stable/ussuri branch as latest stable and
d-g test only latest stable branch to make sure the compatibility
of legacy jobs.

This commit moves the legacy stable job testing to stable/ussuri.

Depends-On: https://review.opendev.org/#/c/723605/
Change-Id: Id57e5590a5b24c2d821381f52e1c445cbbd1105b

playbooks/legacy/tempest-neutron-full/run.yaml

@@ -52,7 +52,7 @@
           # be enough to make sure devstack-gate keep working for
           # stable branch legacy jobs which are not yet migrated
           # to Zuul v3 native jobs.
-          export BRANCH_OVERRIDE=stable/train
+          export BRANCH_OVERRIDE=stable/ussuri
           if [ "$BRANCH_OVERRIDE" != "default" ] ; then
               export OVERRIDE_ZUUL_BRANCH=$BRANCH_OVERRIDE
           fi