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

roles/run-tempest/README.rst

@@ -1,5 +1,8 @@
 Run Tempest
 
+The result of the tempest run is stored in the `tempest_run_result`
+variable (through the `register` statement).
+
 **Role Variables**
 
 .. zuul:rolevar:: devstack_base_dir

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 }}"