Allow overridding OS_TEST_TIMEOUT via the role variables

Change-Id: I7e464ba27704a0b4b13d6e0787b81750deee965e
diff --git a/roles/run-tempest/tasks/main.yaml b/roles/run-tempest/tasks/main.yaml
index 16086aa..24bd4db 100644
--- a/roles/run-tempest/tasks/main.yaml
+++ b/roles/run-tempest/tasks/main.yaml
@@ -42,3 +42,4 @@
     chdir: "{{devstack_base_dir}}/tempest"
   become: true
   become_user: tempest
+  environment: '{{ {"OS_TEST_TIMEOUT": tempest_test_timeout} if tempest_test_timeout else {} }}'