Add tripleo standalone job

A recent change[1] in puppet-oslo breaks the tripleo
jobs.

Adding tripleo-ci-centos-8-standalone job in
check and gate against puppet-oslo project make sure it
does not break the tripleo project.

[1.] https://review.opendev.org/#/c/748067

Related-Bug: #1893173

Change-Id: I5016e565a149595e9904d49e8249e1bbc59c21e0
Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>

.zuul.yaml

@@ -5,3 +5,10 @@
       - puppet-openstack-integration-jobs-all
       - puppet-openstack-beaker-jobs
       - release-notes-jobs-python3
+    check:
+      jobs:
+        - tripleo-ci-centos-8-standalone
+    gate:
+      jobs:
+        - tripleo-ci-centos-8-standalone
+      queue: tripleo