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/README.rst b/roles/run-tempest/README.rst
index 91b0b5f..3643edb 100644
--- a/roles/run-tempest/README.rst
+++ b/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