run-tempest: save the result of tempest execution

This is useful when a job needs to execute additional tests
after tempest run, so ignoring the tempest run failures,
and combine the results afterwards.

Change-Id: I61fc5ebf69db47a035ae8294cfda4047d9f7e21a
diff --git a/roles/run-tempest/tasks/main.yaml b/roles/run-tempest/tasks/main.yaml
index 8686f9a..3e5772b 100644
--- a/roles/run-tempest/tasks/main.yaml
+++ b/roles/run-tempest/tasks/main.yaml
@@ -55,6 +55,7 @@
             --black-regex={{tempest_black_regex|quote}}
   args:
     chdir: "{{devstack_base_dir}}/tempest"
+  register: tempest_run_result
   become: true
   become_user: tempest
   environment: "{{ tempest_tox_environment }}"