Allow overridding OS_TEST_TIMEOUT via the role variables

Change-Id: I7e464ba27704a0b4b13d6e0787b81750deee965e
diff --git a/roles/run-tempest/defaults/main.yaml b/roles/run-tempest/defaults/main.yaml
index 06918b5..79df3e1 100644
--- a/roles/run-tempest/defaults/main.yaml
+++ b/roles/run-tempest/defaults/main.yaml
@@ -3,3 +3,4 @@
 tox_envlist: smoke
 tempest_black_regex: ''
 tox_extra_args: ''
+tempest_test_timeout: ''