Use centos-8 standalone puppet parent

https://review.opendev.org/#/c/711320/ adds
tripleo-puppet-ci-centos-8-standalone job in poi and let's
re-use the same parent for tripleo standalone job.

Change-Id: Idf259de1c9599568ed39774f3b0e6e060310873f
Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>

.zuul.yaml

@@ -14,7 +14,7 @@
 
 - job:
     name: puppet-keystone-tripleo-standalone
-    parent: tripleo-puppet-ci-centos-7-standalone
+    parent: tripleo-puppet-ci-centos-8-standalone
     description: |
       TripleO standalone Job running Identity specific Tempest tests.
     vars: