Explicitly pin openstack-tox to xenial for <=rocky
Explicitly pin openstack-tox to xenial for <=rocky

In preparation for changing our base nodeset to ubuntu-bionic, be
explicit about running stable/ocata through stable/rocky branches on
ubuntu-xenial in a variant of the openstack-tox abstract job and
limit the previous default nodeset variant to branches which aren't
those.

Change-Id: Iac7884dd570aaaa72f088fa0287345feae0976f0

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs