[stf-run-ci] Explicitly check the validate_daployment was successful

In [1], the validate_deployment step is successful, despite the
deployment not being successful.
This causes the job to timeout because the following steps continue to
run despite an invalid state.

To get the expected behaviour, the output should be checked for a string
indicating success.
i.e. * [info] CI Build complete. You can now run tests.
[2] shows the output for a successful run.

[1] https://review.rdoproject.org/zuul/build/245ae63e41884dc09353d938ec9058d7/console#5/0/144/controller
[2] https://review.rdoproject.org/zuul/build/802432b23da24649b818985b7b1633bb/console#5/0/82/controller

build/stf-run-ci/tasks/main.yml

@@ -220,7 +220,8 @@
     args:
       executable: /bin/bash
     register: validate_deployment
+    failed_when: validate_deployment.stdout_lines[-1] != "* [info] CI Build complete. You can now run tests."
 
   - name: Show the result of the validate_deployment script
     ansible.builtin.debug:
-      var: validate_deployment.stdout
+      var: validate_deployment.stdout_lines[-1]