Allow overridding OS_TEST_TIMEOUT via the role variables

Change-Id: I7e464ba27704a0b4b13d6e0787b81750deee965e
diff --git a/roles/run-tempest/README.rst b/roles/run-tempest/README.rst
index e1787b6..d4b253a 100644
--- a/roles/run-tempest/README.rst
+++ b/roles/run-tempest/README.rst
@@ -67,3 +67,8 @@
        ::
            vars:
              tox_extra_args: --sitepackages
+
+.. zuul:rolevar:: tempest_test_timeout
+   :default: ''
+
+   The timeout (in seconds) for each test.