Not load tempest_test_blacklist when tempest_run_skipped is true

We want to run only the tests in skiplist, so skiplist become the
whitelist. If we load both whitelist and skiplist, it will anulate each
other and no tests will be executed.

Change-Id: I99df5e074fea58824c2550abf7b48726cc3564a2

playbooks/tempest.yml

@@ -89,7 +89,9 @@
 - name: Set tempest_test_blacklist
   set_fact:
     tempest_test_blacklist: "{{ known_failures | json_query('[?undercloud != `true`]') }}"
-  when: tempest_cloud_name != 'undercloud'
+  when:
+    - tempest_cloud_name != 'undercloud'
+    - not tempest_run_skipped | default(false) | bool
 
 - name: set python_cmd
   set_fact: