Run lower-constraints job on Xenial

The openstack-lower-constraints-jobs was updated to run in Ubuntu
Bionic, but the underlying version of openssl changed, causing tests
in keystonemiddleware to fail with the current version of
python-keystoneclient:

  https://review.openstack.org/#/c/406175/

Instead of bumping the version immediately, we can ensure the
lower-constraints job runs on Xenial for the time being, making it so
we can backport this fix to stable/stein. A subsequent patch will
update the job to use Bionic when we bump the minimum version of
python-keystoneclient.

Change-Id: I5a5ad8ad86df80755a304f70597578b7dfec2068

.zuul.yaml

@@ -1,7 +1,11 @@
+- job:
+    name: openstack-tox-lower-constraints-keystone-xenial
+    parent: openstack-tox-lower-constraints
+    nodeset: openstack-single-node-xenial
+
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-lower-constraints-jobs
       - openstack-python-jobs
       - openstack-python36-jobs
       - openstack-python37-jobs
@@ -10,3 +14,10 @@
       - lib-forward-testing
       - lib-forward-testing-python3
       - release-notes-jobs-python3
+    check:
+      jobs:
+        - openstack-tox-lower-constraints-keystone-xenial
+    gate:
+      jobs:
+        - openstack-tox-lower-constraints-keystone-xenial
+