Trigger Validation on standalone job with tripleo CLI command

In order to test the TripleO CLI for Validations and the whole chain to avoid
blocker and regression, this patch enable Validation at the end of the Standalone job.

The cost of it, is arround 3 or 4 minutes max.

Change-Id: Ide76602bc4ccf217af00678485d8ea5c5f462bfe

zuul.d/layout.yaml

@@ -31,3 +31,8 @@
               - ^tripleoclient/config/.*minion.*$
               - ^tripleoclient/v1/.*minion.*$
               - ^tripleoclient/v1/tripleo_deploy.py$
+        - tripleo-ci-centos-8-standalone:
+            vars:
+              enable_validation: true
+              validation_component: validation
+              command: openstack tripleo validator