Set tempest_test_extra_test variable

This variable on os_tempest allow us to run an alternative test on
tempest. This is required because very often the included list of tests
is empty because of the match with the excluded list.

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible-os_tempest/+/796818

Change-Id: I95128a9b3e0f6fd1b2c3ddf5b502f460969214b1

playbooks/tasks/tempest.yml

@@ -198,6 +198,7 @@
         tempest_service_setup_host: '{{ inventory_hostname }}'
         tempest_workspace: "{{ ansible_user_dir }}/tempest"
         stackviz_venv_bin: "{{ ansible_user_dir }}/stackviz_venv/bin"
+        tempest_test_extra_test: ['keystone_tempest_plugin.tests.api']
 
     - name: set facts while running tempest from source
       set_fact: